Automatically calibrated RF envelope detector circuit

ABSTRACT

A method and apparatus is disclosed for automatically maintaining a transducing head on the proper track and is particularly adapted for a helical scan recording and reproducing apparatus that is capable of providing special motion effects, such as slow, still and fast motion, and other effects. The apparatus is of the type which utilizes transverse positioning of the transducing head to accurately follow a track during reproducing and, at the completion of the reproduction from the track, to properly position the head in position to either reproduce the next adjacent successive track, reproduce the same track or reproduce another track so that the appropriate special motion effect is achieved. To provide accurate head tracking, a small oscillator motion (dither) is applied to the head to cause it to vibrate laterally between two limits relative to the track. This causes amplitude modulation of the reproduced signal, which is in the form of an RF envelope of frequency modulated carrier. A feedback servo circuit is responsive to the amplitude modulated reproduced signal to provide continuous adjustment of the transducing head position so as to maintain the magnetic head in optimum reproduce relationship with respect to the track. An automatically calibrated RF envelope detector circuit includes a reference level setting feedback loop which takes a reference level from the input signal to the detector circuit to control the gain of the latter circuit regardless of tape RF level differences, operating conditions, component characteristics, etc.

REFERENCE TO RELATED APPLICATIONS AND PATENTS

Hathaway et al., application Ser. No. 677,815, filed Apr. 16, 1976,entitled "Method and Apparatus for Producing Special Motion Effects inVideo Recording and Reproducing Apparatus", and assigned to the sameassignee as the present invention.

Ravizza, application Ser. No. 669,047, filed Mar. 22, 1976, now U.S.Pat. No. 4,151,570, entitled "Automatic Scan Tracking Using a MagneticHead Supported by a Piezoelectric Bender Element", and assigned to thesame assignee as the present invention.

Ravizza, application Ser. No. 677,828, filed Apr. 16, 1976, now U.S.Pat. No. 4,106,065, entitled "Drive Circuitry for Controlling MovableVideo Head", and assigned to the same assignee as the present invention.

Ravizza, application Ser. No. 677,827, filed Apr. 16, 1976, now U.S.Pat. No. 4,080,636, entitled "System for Damping Vibrations in aDeflectable Transducer", and assigned to the same assignee as thepresent invention.

Brown, application Ser. No. 677,683, filed Apr. 16, 1976, now U.S. Pat.No. 4,093,885, entitled "Transducer Assembly Vibration Sensor", andassigned to the same assignee as the present invention.

Mauch, application Ser. No. 874,739, filed Feb. 3, 1978, entitled"Method and Apparatus for Controlling the Movement of a RecordingMedium", and assigned to the same assignee as the present invention.

Ravizza, application Ser. No. 889,451, filed concurrently herewith,entitled "Continuous Slow Motion Automatic Tracking System", andassigned to the same assignee as the present invention.

Ravizza, application Ser. No. 889,994, filed concurrently herewith,entitled "Automatically Compensated Movable Head Servo Circuit andMethod", and assigned to the same assignee as the present invention.

Ravizza, application Ser. No. 889,995, filed concurrently herewith,entitled "Movable Head Automatic Position Acquisition Circuit", andassigned to the same assignee as the present invention.

Ravizza, application Ser. No. 889,461, filed concurrently herewith,entitled "Track Selection Method and Apparatus", and assigned to thesame assignee as the present invention.

DESCRIPTION

The present invention generally relates to improvements in magneticrecording and reproducing apparatus, and more specifically totransducing head servo apparatus to accurately position a head relativeto a recorded track.

In the first five above-identified cross referenced relatedapplications, and, particularly, the Hathaway et al., application Ser.No. 677,815, recording and reproducing apparatus as well as methods aredisclosed which represent significant improvements in achieving superiorrecording and reproducing of video signals whereby special motioneffects are obtained. While the apparatus disclosed therein isapplicable to various alternative types of equipment and is not limitedto recording and reproducing video signals, the apparatus isadvantageously adapted for recording and reproducing video signals onmagnetic tape. This is because the apparatus can reproduce signals in amanner whereby normal speed reproducing, as well as special motioneffects, such as slow and stop motion and faster than normal motion canbe produced without experiencing a noise band or picture breakup in thevideo display. There are many different formats that have been developedin magnetic tape recording and, as described in the above-identifiedHathaway et al application, the recording format that results fromtransporting tape in a helix around a cylindrically shaped drum guide asit is scanned by a transducing head has exhibited many distinctadvantages in terms of relative simplicity of the tape transport driveand control mechanism, the necessary electronics involved, the number oftransducing heads in the apparatus, and the efficient use of magnetictape in terms of the quantity of tape that is required to record a givenamount of information. By helically wrapping the tape around a drumguide, a single transducing head mounted on a rotating drum guide can beutilized for recording and reproducing information. When a single headis used in a helical scan tape recording apparatus, there are two widelyused alternative configurations of guiding (i.e., wrapping) the tapearound the cylindrical drum guide for scanning by the head. They aregenerally referred to as the alpha wrap and the omega wrap types ofhelical scan apparatus. Both wrap configurations involve guiding thetape generally in a helix around the drum guide with the tape exitingthe drum surface at a different axially displaced position relative toits entry position. In other words, if the drum is vertically oriented,the tape leaves the drum surface either higher or lower than when itfirst contacts the surface. The video or other data information signalsare recorded along discrete parallel tracks that are positioned at asmall angle relative to the length of the tape so that a track lengthgreatly exceeds the width of the tape. The angular orientation of therecorded tracks are a function of both the speed of the tape beingtransported around the drum guide as well as the speed of rotation ofthe scanning head. The resultant angle therefore varies depending uponthe relative speeds of the rotating scanning head and the tape beingtransported.

It should be appreciated that the information signals are recorded on atape at a predetermined angle that results from precise rotationalscanning head and tape transport speeds, and that the subsequentreproducing of the information signal should be performed at these samespeeds or the transducing head will not follow the track with precision.If the tape speed is changed during reproducing, i.e., it is reduced oreven stopped, the transducing head will no longer precisely follow therecorded track and may cross onto an adjacent track. The failure toprecisely follow the track in registry during playback results in crosstracking noise and other undesirable signal effects that appear in therepresented information, such as the video picture, in the event videoinformation is being reproduced. While various prior art systems havebeen proposed to reduce the undesirable effects due to the lack ofprecise head-to-track registry such systems have not been entirelysuccessful even at speeds that are intended to be identical to thosethat were used during recording.

Helical tape recorders that are adapted to create special altered timebase reference effects have not been particularly successful to datebecause of the spurious noise that is generated during playback due tothe transducing head crossing from one track to another. For example,slow motion effects and video recording necessarily require that thedata on a track, typically a full video field on each track, be repeatedone or more times during playback so that the visual motion is sloweddown. If data is recorded without redundancy, a track must be reproducedone or more times to accomplish this and hence the tape speed must beslowed. The resultant path that the transducing head follows along thetape during such reproduction processes will therefore be substantiallydifferent than the recorded track that was made during the recordingprocess. A more extreme difference is found in stop motion or stillframe operation, where the tape transport is stopped and the video headscans the same portion of the tape a number of times. During stop motionoperations, the scanning head can cover a portion of the tapecorresponding to that occupied by the two or more adjacent tracks ofrecorded information. To reduce the disturbing effects of noise bars indisplayed video still frames, it has been the practice to adjust thetape position relative to the location of the scanning head so that thehead begins and ends each tape scan in the guardbands adjacent to thedesired track and scans the desired track during the intermediateinterval of each tape scan. This places the visual disturbance noisebars at the top and bottom of the displayed video still frame, leavingthe center of the displayed video relatively free of disturbing effects.

While techniques have been proposed to reduce or overcome the noise barthat is generated by crossing tracks, such techniques have not beenparticularly successful until the advent of the apparatus described inthe first five above-identified cross referenced applications,particularly, Hathaway et al., Ser. No. 677,815. As is comprehensivelyset forth therein, the method and apparatus automatically positions atransducing head to accurately follow a desired path along a magnetictape and to rapidly position the transducing head, if necessary, at thebeginning of the path that is desired to be followed next. The nexttrack that is to be followed, whether during reproducing or recording,is a function of the mode of operation that is selected. From theplayback of video signals, the various modes may include a slow andstill motion effect mode, a speeded up or fast motion effect mode, and areverse motion effect mode. Other modes of operation may include skipfield recording and compensation playback mode as well as a surveillancemode. In both of the latter modes, the period of time that can berecorded on a given length of tape is greatly increased by skipping oneor a number of fields during the recording operation, such as recordingevery other field or one of every sixty fields, for example. Theapparatus permits the tracks to be accurately followed even though thetransport speed of the tape can vary within wide limits. In the eventfast motion effects are to be achieved during playback of video signals,the transport speed of the tape must be increased and, conversely, forslow motion effects the transport speed must be slowed. For the stopmotion effect, one field is typically reproduced many times over and, insuch mode, the tape is not moving at all, the relative motion betweenthe tape and the transducing head being supplied by the rotation of arotating drum guide carrying the head. Changing the tape transport speedchanges the angle of the path followed by the head along the tape.Consequently, if the video transducing head carried by the rotating drumguide is maintained in a fixed position relative to the drum, it can notexactly follow a previously recorded track when the transport speed ofthe tape is altered during reproduction relative to its speed duringrecording.

The apparatus disclosed in the first five above-identifiedcross-referenced applications employs means that move the transducinghead transversely relative to the longitudinal direction of the tracksso that the head follows selected tracks along the magnetic tape and,thereafter, selectively alters or changes the positon of the head afterthe head completes the scan of a selected track so as to correctlyposition the head to commence following another track. In the event thehead is to follow the next adjacent downstream track, the head would bein the correct position to begin following it at the completion of thescan of a previously selected track. It should be understood that onecomplete revolution of the transducing head causes the head to scan atrack at a predetermined angular orientation relative to the length ofthe tape and, at the end of the revolution, the movement of the tapecauses the head to be gradually displaced a predetermined distancedownstream of the tape in position to begin scanning the next adjacenttrack. In this manner, the head, for example, during recordingoperations, records information along tracks that are parallel to oneanother and, assuming the transport speed of the tape and the speed ofrotation of the scanning head are maintained constant, the tracks willhave a constant spacing relative to adjacent tracks, i.e., the center tocenter distance between adjacent tracks will be substantially constantin the absence of geometric errors. Geometric errors are introduced bytemperature or humidity induced dimensional changes of the tape, byfaulty tensioning mechanism in the tape transport that causes stretchingof the tape, or by imperfect control of the relative head to tape speed.During normal speed playback operations, i.e., the tape is being movedand the head is being rotated at the same speeds as they were during therecording operation, the scanning head will follow a track during asingle revolution and be in position to begin following the nextadjacent downstream track during the next revolution. Furthermore, eachtrack will be followed once and produce unaltered time base effects aswould be expected, such as normal speed visual effects of recorded videoinformation. In the event it is desired to produce a still frame or stopmotion effect, the transport of the tape is stopped and one recordedtrack is typically repeated indefinitely. In this mode of operation, thetransducing head will be continuously deflected to follow the track frombeginning to end and, at the end, the head will be reset in thedirection opposite the direction it has been deflected to position it atthe beginning of the same track. The distance that the head is deflectedfrom its normal path as it scans the track, and subsequently reset, itequal to the center to center spacing between adjacent tracks. Thus, bycontinuously deflecting the head to follow a track, resetting the headand deflecting the head again to follow the same track, a single fieldis repetitively reproduced, thereby permitting a stop motion or stillframe visual picture to be displayed. This will be more comprehensivelydescribed herein with respect to certain figures of the drawings and iscomprehensively described in the aforementioned Hathaway et alapplication, Ser. No. 677,815.

The position of the transducing head relative to the data track ismonitored during the reproduce process over the entire scan of each datatrack while applying a small transverse oscillatory motion (dither) tothe head via a supporting positionable element, to cause the head tovibrate laterally relative to its normal scanning path. This vibrationor dither signal is detected and is employed to amplitude modulate thereproduced data signal's RF envelope, wherein the change in amplitudemodulation is indicative of the amount of lateral displacement of thetransducing head from its optimum position on a track. The direction ofdisplacement is reflected in the phase of the envelope amplitudemodulation at the fundamental frequency of the dither signal.

To maintain the transducing head continuously in the optimum reproducingposition, the polarity and amplitude of the modulated RF envelope isdetected and a correction signal indicative of the head displacementfrom track center is generated and is fed back via a tracking servo toadjust, and thus control, the movement of the positionable element andthus the position of the head relative to the track.

Since the amplitude modulation of the RF envelope is indicative of thelateral displacement of the head, the accuracy of the measurementdepends upon how free the RF envelope of the reproduced signal is fromnon-representative inputs which affect the RF modulation. Typical ofsuch inputs are variations in the RF level caused by differences in tapeformulations, differences between heads, head and tape wear, variationsin head to tape contact, etc. The RF level variations causecorresponding variations in the amplitude of the modulated RF envelopewhich, in turn, cause inaccurate measurement of the detected amplitudesand inconsistent envelope detector circuit output signals. In addition,envelope detector integrated circuits such as employed in the trackingservo to detect the RF envelope amplitudes, exhibit varyingsensitivities and DC offset characteristics from chip-to-chip, whichalso causes inaccurate measurement of the detected amplitudes.

Prior art tracking servos employ a manually controlled RF leveladjusting circuit in the form of a potentiometer, whereby during thereproduce process the RF level is detected and is manually adjustedaccordingly to compensate for variations between tapes, reproducingapparatus, etc. Such manual adjustment is cumbersome and time consuming,and fails to compensate for the substantial differences in envelopedetector chip sensitivities and DC offset characteristics. Method andapparatus that is capable of operating in various signal reproducingmodes, including slow/still motion mode, reverse mode, and regularmotion mode and which is uniquely adapted to be switched from one modeto another without producing disturbing effects in the displayedinformation in the event that information is being reproduced.

Yet another object of the present invention is to provide a method andapparatus for recording and reproducing information with respect to atape recording medium using rotary scan tape recording equipment whichenables information to be transferred with respect to the tape mediumduring changes in the relative head to tape speeds without theintroduction of disturbing transients into the transferred information.

Other objects and advantages will become apparent upon reading thefollowing detailed description, while referring to the attacheddrawings, in which:

FIG. 1 is an electrical block diagram illustrating automatic headtracking servo control circuitry in a recording and reproducingapparatus, as generally disclosed in the aforementioned Hathaway et alcross referenced application, Ser. No. 677,815;

FIG. 2 is a block diagram of circuitry in a recording and reproducingapparatus, the portions shown in the dotted line box being adapted forsubstitution in the circuitry shown within the dotted line box of FIG.1;

FIG. 3 is a more detailed electrical block diagram of the circuitryshown in FIG. 2,

FIG. 4 is a perspective view of the helical tape guide and scanning headassembly portion of an omega wrap helical scan recording and/orreproducing apparatus which is simplified for the sake of clarity andwhich can be used together with the present invention;

FIG. 5 is a side elevation of the drum tape guide and scanning headassembly shown in FIG. 1, with portions removed and partially in crosssection;

FIG. 6 is an enlarged segment of magnetic tape having tracks A-Grecorded thereon;

FIG. 7a is a diagram illustrating the voltage amplitude versus timecharacteristic of a typical RF envelope and having time exaggerated dropout areas, which diagram may be produced using the drum and headassembly shown in FIGS. 4 and 5 on the magnetic tape shown in FIG. 6;

FIG. 7b is a diagram illustrating a typical voltage waveform that may beproduced to provide the desired head deflection of the reproduce headshown in FIGS. 4 and 5 when the apparatus is in the slow/still mode andthe transport of the tape is stopped;

FIG. 7c is a diagram of the time versus amplitude of the head deflectionwaveform for the slow/still motion mode and illustrates the operation ofcircuitry disclosed in the aforementioned Hathaway et al application,Ser. No. 677,815;

FIG. 7d is a diagram of time versus amplitude of the head deflectionwaveform for a slow motion operation and illustrates the operation ofcircuitry incorporated in the apparatus when in the slow/still motionmode;

FIG. 7e is a diagram of time versus amplitude of the head deflectionwaveform for a slow motion operation and illustrates the operation ofthe apparatus when in the 95% of normal speed mode;

FIG. 7f is a diagram of time versus amplitude of the head deflectionwaveform during acquisition of the proper track and for a subsequentnormal speed operation and illustrates the operation of the apparatuswhen in the normal speed mode of operation;

FIG. 7g is a diagram of time versus amplitude of the head deflectionwaveform for a 2 times normal speed operating and illustrates theoperation of the apparatus when in the 2 times normal speed mode.

FIG. 8 is a block diagram of the capstan tach and control track servocircuitry portions of the present invention;

FIG. 9 is a diagram illustrating the tape velocity versus time profilethat is produced by the capstan tach and control track servo circuitryshown in FIG. 8;

FIG. 10 is a unitary diagram illustrating orientation of the sheetscontaining FIGS. 10a and 10b;

FIGS. 10a and 10b together comprise a detailed electrical schematicdiagram illustrating circuitry that may be used to carry out theoperation of the block diagram of FIG. 3 as well as certain portions ofthe block diagram shown in FIG. 1;

FIGS. 10c and 10d illustrate electrical schematic diagrams ofmodifications of the circuitry shown in FIGS. 10a and 10b that may beused to control still frame modes during which more than one televisionfield is reproduced to generate still frame displays;

FIG. 11 is a unitary diagram illustrating orientation of the sheetscontaining FIGS. 11a, 11b and 11c;

FIGS. 11a, 11b and 11c together comprise a detailed schematlc diagram ofcircuitry that can be used to carry out the operation of the capstantach servo circuitry portion of the block diagram shown in FIG. 8;

FIG. 12 is an electrical block diagram illustrating the automatic headtracking servo control circuitry in a recording and/or reproducingapparatus employing the present invention;

FIG. 13 is a schematic block diagram of the automatically compensatedmovable head tracking servo control circuitry;

FIGS. 14a-14f are timing diagrams illustrating operation of theautomatic movable head tracking servo control circuitry shown in FIG.13;

FIG. 15 is a frequency spectrum diagram illustrating selection of thedither frequency so as to avoid spectrum overlap; and

FIG. 16 is a timing diagram illustrating operation of the trackselection logic.

Before describing the method and apparatus that embodies the presentinvention, the environment in which the present invention can beutilized will initially be broadly described so as to provide a betterunderstanding of the present invention. While the aforementionedHathaway et al U.S. Pat. No. 4,151,570, as well as Ravizza application,Ser. No. 669,047, comprehensively sets forth the background and theenvironment to which the present invention can be applied, a briefdescription of the environment will be set forth herein. Also, while thepresent invention is particularly adapted for use with helical scantypes of video tape recorders, it should be understood that the presentinvention is not limited to helical recorders and may be used withquadrature, segmented helical, arcuate and other types of rotary scanvideo tape recorders. In addition, the present invention is suited foruse with various tape recording formats characteristic of the variousrotary scan tape recorders. Furthermore, the present invention is notlimited to use in rotary scan tape recorders designed for processingvideo signals. It is contemplated that the present invention will findutility in any application where it is desired to record or reproduce,i.e., transfer information with respect to a tape recording mediumwithout the introduction of disturbing transients into the transferredinformation while the relative head-to-tape speed undergoes changes.

Turning now to the drawings, and particularly FIGS. 4 and 5, there isshown a helical video scanning head and cylindrical tape guide drumassembly indicated generally at 20, with FIG. 5 showing portions brokenaway. The head-drum assembly 20 is shown to comprise a rotatable upperdrum portion 22 and a stationary lower drum portion 24, the upper drumportion 22 being fixed to a shaft 26 which is rotatably journaled in abearing 28 that is mounted on the lower drum 24, the shaft 26 beingdriven by a motor (not shown) operatively connected thereto in aconventional manner. The head-drum assembly 20 has a video transducinghead 30 carried by the rotatable drum portion 22 and is shown to bemounted on an elongated movable support element 32 that is in turnmounted at one end in a cantilever type support 34 that is fixed to theupper drum portion 22. The element 32 is preferably of the type thatflexes or bends in a direction transversely of the recorded track withthe amount and direction of movement being a function of the electricalsignals that are applied to it.

As is best shown in FIG. 4, the head-drum assembly 20 is part of ahelical omega wrap video tape recorder which has the magnetic tape 36advancing toward the lower drum 24 in the direction of the arrow 38 asshown. More specifically, the tape is introduced to the drum surfacefrom the lower right as shown in the drawing and is fed around a guidepost 40 which brings the tape into contact with the outer surface of thestationary lower drum portion 24 whereupon the tape travelssubstantially completely around the cylindrical drum tape guide until itpasses around a second guide post 42, which changes the direction of thetape as it exits the head-drum assembly 20.

As is best shown in FIGS. 4 and 6, the configuration of the tape path issuch that the tape 36 does not contact the guiding drum surface over afull 360 degree rotation because of the clearance that is required forentrance and exit of the tape. This gap preferably does not exceed adrum angle of more than about 16 degrees which has the effect ofcreating a drop out interval of information. In the case of recordingvideo information, the occurrence of the drop out is preferably chosenrelative to the video information being recorded so that the informationthat is lost does not occur during the active portion of the videosignal and, in the case of recording and reproducing video signals, sothat the start of the scan of a track can be properly field synchronizedto the video signal.

The transducing head 30 is mounted upon the elongated movable,preferably flexible, element 32 which may comprise an elongated twolayer element (sometimes referred to as a bimorph) that exhibitsdimensional changes in the presence of an electric or magnetic field.The deflectable, movable element 32 is effective to move the transducinghead 30 mounted thereto in a vertical direction as shown in FIG. 5 inaccordance with the electrical signals that are applied throughconductors 44 from the automatic head tracking servo circuitryschematically illustrated by a block 46. The head 30 is mounted toextend slightly beyond the outer surface of the rotating drum portion22, the head extending through an opening 48 in the outer surfacethereof. The movable element 32 is adapted to sweep or bend and displacethe transducing head along a path that is transverse to the direction ofrelative motion of the head 30 with respect to the magnetic tape 36,i.e., transverse to the direction of the recorded tracks.

If the transport speed of the magnetic tape 36 is changed during thereproducing of recorded information, relative to the speed at which theinformation was recorded on the tape, then the angle of the path scannedby the head 30 relative to the length of the tape 36 is changed and headpositioning error correcting signals will be produced for the purpose ofhaving the transducing head follow the track of recorded informationwhich is at the different angle. Since the movable element 32 is movablein either direction, the tape can be transported around the tape guidedrums 22, 24 at either a faster or slower speed relative to therecording speed and the movable element can position the head 30 tofollow the recorded track for either condition.

Referring to FIG. 6, there is illustrated a segment of magnetic tape 36having a number of tracks A-G thereon as may be recorded by thetransducing head 30 as the tape is transported about the guide drums 22,24 shown in FIG. 4. The segment of tape is shown to have an arrow 38which illustrates the direction of tape movement around the drum and anarrow 50 which shows the direction of the scanning head movementrelative to the tape. Thus, when the upper portion 22 rotates in thedirection of the arrow 50 (FIG. 4), the transducing head 30 moves alongthe tape in the direction of the arrow 50' shown in FIG. 6. With aconstant transport speed of the tape 36 and angular velocity of therotating drum portion 22, tracks A-G will be substantially straight andparallel to one another at an angle θ (of about 3°, for example)relative to the longitudinal direction of the tape, with each rightwardtrack shown in the drawing being successively produced during arecording operation. Since track B, for example, would be recordedimmediately after track A was recorded during constant drum and headrotation and tape transport speeds, it should also be appreciated thatif these speeds are maintained during the reproducing or playbackoperation, the transducing head 30 would play back track B during asuccessive revolution immediately after having reproduced theinformation from track A.

If conditions were ideal and no tape transport disturbance wasintroduced, then the transducing head 30 would simply successivelyfollow the adjacent tracks without adjustment, because no error signalswould be produced for transversely moving the transducing head 30relative to the track. Stated in other words, the transducing head isautomatically in position to begin reproducing the subsequent track Bafter completing the reproducing of the information from track A. Itshould also be appreciated that even if the tape transport speed isvaried during reproducing relative to the tape transport speed duringrecording and the head is transversely moved to maintain accurate headtracking during reproduction of the track, then at the end of the head'sscanning of a track being reproduced, the head is nevertheless in aposition to begin reproducing the next adjacent downstream track, i.e.,track B in the event reproduction of track A was completed. This occurseven when the tape is stopped or is traveling slower or faster than thetransport recording speed.

To achieve special motion and other effects during reproduction of theinformation signals that are recorded on a tape, it is necessary to varyor adjust the transport speed of the tape past the location of thescanning head, hence, around the tape guide drums 22, 24 in theillustrated embodiment. To produce a speeded up or fast motion effect,the transport speed is increased during reproducing relative to thatwhich was used during the recording process. Similarly, to produce slowmotion effects, it is necessary to reduce the speed of the transporttape around the tape guide drums during reproducing relative to thatwhich was used during the recording process. In stop motion modes thetape is stopped during reproducing so that the rotating transducing head30 can repetitively reproduce the signals, typically from a singlerecorded track.

The apparatus disclosed in the aforementioned Hathaway et alapplication, Ser. No. 677,815, can be placed in different modes ofoperation wherein either forward or reverse motion effects are achievedand the motion can be speeded up or slowed down by simply adjusting thetransport speed of the tape in such forward or reverse directions toobtain the desired speed of motion upon reproducing the recordedinformation. Once a motion direction is chosen, the apparatuseffectively automatically positions the transducing head to follow atrack from beginning to completion and to thereafter adjust the positionof the transducing head (if adjustment is needed) to the beginning ofthe proper track. The apparatus automatically provides for transverselymoving or resetting the transducing head 30 at the end of the head scanof a track to a position corresponding to the start of a track otherthan the next successive adjacent track under certain predeterminedconditions and not transversely moving or resetting the transducing headunder other conditions. The decision to transversely adjust the positionof the transducing head depends upon the mode in which the apparatus isoperating and whether the amount of transverse movement is within thepredetermined limits that can be achieved. If the transducing head 30 isdeflected the maximum amount in one direction permitted by the movableelement 32, it cannot be moved further in that direction. The totalrange of movement shall be within the practical limits determined by thecharacteristics of the movable element 32.

When the apparatus is in the slow motion or still frame mode ofoperation, the transducing head 30 may be required to be reset at thecompletion of the scan by the head of the track being reproduceddepending upon whether the deflection of the transducing head reachesthe predetermined threshold limits set for the displacement of theelement 32 at the completion of a track. When the tape 36 is stopped soas to provide still frame or stop motion, the transducing head 30 istypically reset at the completion of the scan by the head of the trackbeing reproduced and is thereby reset to the beginning of that track sothat its scan can be repeated by the head as many times as is requiredfor the duration of the display of the scene. Thus, the informationrecorded in the track is effectively reproduced over and over since thetape 36 is stationary. Since the transducing head 30 is deflected in thereverse direction relative to the direction the tape is transportedduring a recorded operation to follow the track during each repeatingreproduction, the total deflection in the reverse direction being equalto the track center to track center spacing, d, of the recorded tracks,the head 30 must be reset a corresponding distance in the opposite, orforward direction at the completion of the scan of the track in order tobe correctly positioned to rescan the same track. Since the angle of thepath followed by the head 30 relative to the tape 36 is different whenthe tape is stopped from the angle of recorded tracks, the position ofthe head is also gradually being adjusted in the axial direction of thehead-drum assembly 20 through the course of reproducing the informationsignal on a track. Thus, as the scanning head 30 moves along the track,the head positioning error correcting signals cause it to be movedtransversely to maintain head to track registry and the head is reset atthe end of its scan of the track essentially one track transversedistance, d, in order to be in position for beginning the rescan of thesame track.

To maintain the transducing head 30 in registration with the track as itfollows a track during a revolution of the rotating drum 22, a servocircuit is used which produces an error correcting signal that ispreferably a low frequency or changing DC level and is produced byapparatus such as disclosed in the aforementioned Ravizza application,Ser. No. 669,047. As the head 30 scans a track, the error signal causesthe head to be adjusted so as to follow the track regardless of thespeed of tape transport, provided it is within the limits of movement ofthe element 32.

Referring to FIG. 1, which illustrates a block diagram of circuitrygenerally embodying the apparatus described in the aforementionedRavizza U.S. Pat. No. 4,151,570, and Hathaway et al application, Ser.Nos. 669,047 and 677,815, a dither oscillator 60 applies a sinusoidallyvarying signal of frequency f_(d) on line 62 that is coupled to asumming circuit 64, where it is added to a DC error correction signalfrom line 66. The output of the summing circuit 64 is applied on line 68to a second summing circuit 69 where it is added to the damping signalprovided by an electronic damping circuit 71 over line 73, such asdisclosed in the aforementioned Ravizza U.S. Pat. No. 4,080,636. Asdescribed in that Ravizza application, extraneous disturbing vibrationsin the movable element 32 are detected by the electrically isolatedsense strip 83 proximate an edge of the piezoelectric transducer locatedon one side of the movable element. The sense strip 83 longitudinallyextends along the movable element 32 and is constructed in the mannerdescribed in the aforementioned Brown U.S. Pat. No. 4,093,885. The sensestrip 83 generates a feedback signal representative of the instantaneousdeflection velocity of the movable element and applies the signal toline 77 extending to the input of the electronic damping circuit 71.

The electronic damping circuit responsively generates a damping signalof the proper phase and amplitude for application to the movable elementto oppose and, thereby dampen the extraneous distrubing vibrationspresent therein. The combined error correction signal and damping signalprovided by the second summing circuit 69 is coupled by the line 79 tothe input of a drive amplifier 70 which then provides a signal over aline 81 to the piezoelectric movable element 32 carrying the transducinghead 30. The dither drive signal causes the movable element 32 to imparta small peak-to-peak oscillatory motion (dither) to the head 30 to causethe head to move laterally relative to the track alternately betweenlimits as it scans longitudinally along the track to reproduce therecorded signal. The oscillatory motion imparted to the head 30 causesan amplitude modulation of the reproduced signal which, when recordingvideo or other high frequency signals, is in the form of an RF envelopeof a frequency modulated carrier. The oscillating motion of the movableelement 32 produces an amplitude modulation of the RF envelope. If thehead is located in the center of the track, only even harmonic amplitudemodulation components of the dither signal are produced on the RFenvelope by the action of the movable element 32, because the averagehead position is at track center and the RF envelope variation caused bydithering appears as a symmetrical function. With the head 30 at trackcenter, the amplitude of the RF produced from the tape is maximum. Asthe head 30 moves to either side of track center during each half cycleof the dither signal, the amplitude of the reproduced RF envelopedecreases.

On the other hand, if the transducing head 30 is located slightly offthe center to either side of a track, the reproduced RF envelopeamplitude variation will not be symmetrical because the head 30excursions to one side of the track will produce a different RF envelopeamplitude change than produced by an excursion towards the oppositeside. Hence, a maximum-to-minimum envelope amplitude variation occursonce for each cycle of the dither signal, or at the dither frequency,f_(d), with the order of occurrence of the maximum and minimum envelopeamplitudes depending upon the side of the track center to which the head30 is offset. The fundamental of the dither frequency is no longerbalanced out, and the reproduced RF envelope variations will contain afundamental component of the dither frequency, with the phase of thefundamental component for an offset to one side of the center of a trackbeing 180 degress different with respect to that for an offset to theother side of the center of the track. Detection of the order ofoccurrence of the maximum and minimum envelope amplitudes, i.e., phaseof the envelope amplitude variations, provides information definitive ofthe direction the transducing head 30 is offset from the center of atrack being scanned, and detection of the envelope amplitude variationprovides information definitive of the amount of offset.

To obtain the head position information, the modulated RF envelopesignal reproduced by the head 30 is coupled to detection circuitrythrough a video preamplifier 72 and is applied to equalization circuitry74 before it is coupled by a line 75 to an automatically calibrated,amplitude modulation RF envelope detector circuit 76 which, inaccordance with the present invention, is constructed to recover thedither signal fundamental and its side bands as further described belowwith respect to FIG. 10a. The output of the envelope detector circuit 76is then applied to a synchronous amplitude modulation detector 78. Thesynchronous detector 78 operates on the principle of coherentlydetecting the amplitude and polarity of an unknown actual phase butknown frequency input signal with reference to the phase of a referencesignal of the same nominal frequency. The reference signal is providedby the dither generator 60 through line 62 which is connected to a phaseadjust means 85 and, subsequently to the detector 78. The phase adjustmeans 85 in the VPR-1 video production recorder manufactured by AmpexCorporation is a manually-controlled adjustment that is typically setfor each head and movable element assembly used in a recorder. The phaseof the reference signal is adjusted to compensate for phase changesintroduced to the dither signal by factors other than the transducinghead 30 being located off the center of a track being scanned, such aschanges in mechanical resonance characteristic of the head and movableelement assembly. However, as will be described in detail hereinbelowwith reference to FIGS. 12-15, the apparatus herein utilizes anautomatically phase compensated reference dither signal to avoid thenecessity of having to manually adjust the phase of the dither referencesignal for each video record/reproduce apparatus having a positionablehead that is controlled in the manner accomplished by the apparatusdescribed herein or in the aforementioned Ravizza application, Ser. No.669,047.

The synchronous detector 78 provides a rectified output having theamplitude of the unknown recovered dither signal with the rectifiedoutput being positive when the reference and recovered dither signalsare in phase and negative when the two signals are 180 degrees out ofphase. Since the signal present at the input of the detector from theenvelope detector 76 will have a component at the fundamental ditherfrequency, f_(d), whenever an error occurs in the head track position,the sync detector 78 will provide on its output line 80 a track errorsignal representative of the head track position error. The amplitude ofthe error is proportional to the amount that the head 30 is displacedfrom track center and the polarity of the track error signal isindicative of the direction of head displacement from the track center.The output line 80 is coupled to circuitry 82 shown in the dotted linebox, and the output from that circuitry provides the error correctingsignal on line 66 to the summing circuit 64 as previously described. Inthe event a reset signal is to be produced for resetting the head 30 toa different track upon completion of the scan of a track, it isaccomplished by the circuitry 82.

In the apparatus described in the aforementioned Hathaway et al.application, the circuitry 82 which generates the pulses for changingthe position of the head 30 relative to its location at the conclusionof scanning a track is in part determined by the mode of operation ofthe apparatus, i.e., normal reproduction mode, slow motion mode, etc.,and, in part, by the circuitry which determines the position of the head30 with respect to its range of movement. As can be seen from FIG. 1,the aforementioned Hathaway et al. application has a mode select switch84 that is adapted to bring into operation an upper slow/still servoamplifier circuit 86 or a lower normal play servo amplifier circuit 88,with the mode being determined by the operator using the recordingapparatus. As is evident from the drawing, it is seen that the modeselect switch 84 must be changed from one position to the other whenchanging from normal play to the slow/still mode of operation or fromthe latter to the former. When changing between the normal play and theslow/still modes by the operation of the switch 84, a disturbingtransient interruption occurs in the reproduced video signal because theproper controlling head position error signal is temporarily lost.Reacquisition of the correct controlling error signal can take 100milliseconds or six television fields. It should be appreciated thatthis would produce a discontinuous video picture on a monitor.

Referring to FIG. 2, the circuitry 82 shown in the dotted line box ofFIG. 1 is replaced with the universal circuitry 90 which has input line80 and output line 66 corresponding to the input and output lines of thecircuitry 82 in FIG. 1. The circuitry 90 of FIG. 2 effectively carriesout both the normal play as well as the slow/still modes of operationwith the mode select line 92 controlling the circuitry which replacesthe separate circuits 86 and 88 of FIG. 1. The universal circuitryallows the automatic head tracking servo circuitry to be switched fromthe slow/still mode to normal play mode without producing servounlocking and reacquisition transitions as is experienced by thecircuitry of FIG. 1, when switching between the slow/still servoamplifier circuit 86 and the normal play servo amplifier circuit 88. Thecircuitry of FIG. 2 broadly illustrates that a mode change will notcause the switching out of one circuit and switching in of another and,thereby, does not result in the loss of and necessitate thereacquisition of the error signal. However, it should be appreciatedthat different servo response characteristics are needed for normal playoperations and for slow/still operations; and the circuitry 90 shown inFIG. 2 provides the needed different servo response characteristics.

In addition to the universal automatic head tracking servo circuitry,the improved apparatus includes improved circuitry for controlling themovement of the tape around the tape guide drums 22 and 24, hereinreferred to as the capstan servo. The improved tape transport servoprovides coordinated sequences for changing from a slow/still motionmode of operation to the normal speed mode of operation in a mannerwhereby the automatic tracking servo circuitry can be coordinated toproduce the desired stable, noise free video picture on a monitor, forexample.

The sequence of events that occur during the switching between theslow/still mode of operation and the normal speed mode enablescontinuous video reproducing throughout the period of changing velocitybecause the automatic head tracking servo circuitry operates throughoutthe time in which the tape is moved between a stop or slow motion andthe normal speed motion by the tape transport servo system. As usedherein, normal speed is intended to mean the tape speed that is usedduring recording. When changing from a stop or slow motion operation toa normal speed operation, the tape 36 is accelerated for a period ofabout 1/2 second until it reaches and is moving at a constant speed thatis about 95% of the normal speed. When the tape 36 is moving at 95% ofthe normal speed, the rate at which the tape 36 is transported past thelocation of the scanning head 30 is 5% less than the normal rate. Thisdecrease in the unit length of tape transported past the scanning headlocation per unit time is referred to as tape slippage. It is duringthis time that the initial color frame decision is made. Color framingis the final step in a video record/reproduce system servo operation incorrectly positioning a head to scan a selected track at the properhead-to-tape speed relative to a controlling reference, typically studioreference. In the color framing servo operation, the head and tapepositioning drives are controlled so that recorded video fields arereproduced having a color subcarrier to vertical sync phase relationshipwhich corresponds to that of the studio reference. Because the automatictracking servo circuitry is fully operational during this initial colorframe acquisition time, the video framing information can be evaluatedalong with the reproduced control track data in order to initiallydetermine the color frame. The initial acquisition period varies betweenabout 0.3 and 0.6 second; and, once the initial color framedetermination has been made, the tape transport servo system switches toaccelerate the tape to 100% of normal speed.

It should be understood that a control track 94 (shown in FIG. 6 to bein the longitudinal direction of the tape 36) provides different colorframe information than the actual color frame information obtainablefrom the video information recorded in the tracks A-G as shown in FIG.6. Because of machine-to-machine tolerance variations affecting thelocation of the control track reproduce head 267 (FIG. 8), such as, forexample, variations in the distance separating the control track andmovable video heads and in the mounting of the video head 30 on therotating drum portion 22, it is possible that an initial color framingoperation performed with respect to a comparison of control trackinformation and studio reference will result in positioning the tape 36relative to the location of the movable video head 30 with the headmispositioned as far as plus or minus one (1) track from the propertrack for the correct color frame condition. In other words, instead ofthe video head 30 of the reproduce video tape recorder being positionedto scan the same track that was previously recorded simultaneously withthe detected control track pulse, it is positioned over one of theadjacent tracks because of the aforementioned machine-to-machinetolerance variations although the reproduced control track informationindicates that color framing has been achieved. As will be described ingreater detail hereinbelow, the apparatus described herein includesmeans for automatically verifying that the initial color frameacquisition is correct and, if color frame acquisition is not verified,for automatically relative positioning the video reproduce head 30 andthe tape 36 to place the head over the proper track for achieving colorframe. Thereafter, the tape transport servo maintains the transport ofthe tape 36 phase locked to the reproduced control track signals.

The exemplary embodiment of the apparatus described in theaforementioned Hathaway et al. application, Ser. No. 677,815, utilizeslevel detectors during the slow/still mode of operation to determine ifreset pulses are to be applied to the deflectable piezoelectric element32. In this regard, reference is made to FIG. 7a which illustrates adiagram of the RF envelopes 100 that are produced during successivescanning revolutions, with signal drop out intervals 102 occurring inthe RF envelope which corresponds to the interval that the head 30 isbetween the guides 40 and 42 (FIG. 4) where no tape is present duringthe transducing head's rotation. In FIG. 7a, the drop out intervals 102are exaggerated to facilitate the description. Thus, more specificallywith respect to FIG. 7a, as the rotating head 30 makes a revolution, anRF envelope 100 is reproduced each revolution, with a drop out interval102. When the transducing head 30 is reproducing a track from beginningto end, the RF envelope 100 is produced from the left to the right asportrayed in FIG. 7a, with each area 100 representing the signalinformation that is reproduced or recorded on a single track and, in thecase of video recording, preferably represents at least the completeportion of a field of video information displayed on a monitor. In theevent the apparatus is operating in the slow/still mode of operation andthe tape 36 is stopped so as to be producing a still frame or stopmotion video image on a monitor, it is necessary to reset thetransducing head 30 at the end of its scan of every track, or a sequenceof tracks if a still image monochrome frame or color frame is to berepetitively generated, so that it is in position to repeatedlyreproduce from the same track or sequence of tracks. When such is done,it should be appreciated that the automatic head tracking circuitry willfollow the track during reproducing and will produce a reset pulse forresetting the transducing head 30 at the completion of its scan of thetrack or sequence of tracks. A head deflection voltage versus timewaveform diagram for still frame operation in which a single field isrepetitively reproduced to form the displayed still image is shown inFIG. 7b and includes ramp portions 104 as well as vertical resetportions 106 and generally represents the waveform that is necessary tomaintain head tracking during reproducing of a track and resetting ofthe transducing head 30 at the end of its scan of the track. The timingof the reset is advantageously set in the exemplary embodiment of theaforementioned Hathaway et al. application to occur during the drop outinterval 102 and the amplitude of the reset pulses effecting theresetting of the head 30 depicted by the reset portion 106 of the headdeflection waveform in FIG. 7b is shown to be that which produces atransverse movement of the head 30 that is equal to the center to centerdistance d between adjacent tracks, which will often hereafter bereferred to as a full one track reset. It is advantageous to time theresetting of the movable head 30 with the occurrence of the drop outinterval 102 because that interval typically occurs during the verticalblanking period of the video signal, which provides more than sufficienttime to reposition the movable head 30 before the video image portion ofthe recorded video signal is positioned to be reproduced by the head.However, it is not a requirement of the apparatus constructed inaccordance with the principles described herein that the resetting ofthe movable head 30 be timed to occur during a drop out interval. Forexample, in video record/reproduce apparatus characterized by recordingformats without drop out intervals or with the vertical blanking periodnot aligned with the end of the recorded track, or in data recordingapparatus for signals other than analog video signals, the resetting ofthe head position may be selected to occur during the intermediateportion of a track so that a segment of information is transferred withrespect to the recording medium by a movable head that scans portions ofadjacent tracks and is reset between intermediate locations of theadjacent tracks to rescan the track portions.

However, resetting of the movable head 30 is synchronized to occurduring the drop out intervals 102 that are located at the ends of therecorded tracks. In this regard, level detectors in the circuitry 90effectively monitor the voltage waveform, such as that shown in FIG. 7b,and provide a reset pulse 106 when the voltage near the end of the ramp104 shown at point 108 exceeds a certain level. As shown in FIG. 7, theresetting of the movable head 30 begins at the start of the drop outinterval 102 and is completed before the end of the drop out interval.

In the apparatus described in the aforementioned Hathaway et al.application, the threshold levels for determining whether a headposition reset should occur are shown in FIG. 7c, together with arepresentative head deflection waveform including the ramp portions 104and reset portions 106 shown by the phantom lines. The logic isresponsive to a processed once around drum tach pulse each time the head30 reaches a point in its rotation corresponding to the point 108 inFIG. 7c to provide a single amplitude reset pulse (1 track forwardreset) if the head deflection waveform is at a voltage levelcorresponding to a head deflection in a direction reverse to the travelof the tape 36 past the scanning head location (labeled reverse) and adouble amplitude reset pulse (2 track forward reset) when the voltageexceeds a level corresponding to a head deflection in a directionreverse to the travel of the tape in excess of the spacing betweenadjacent tracks, for example, as depicted ramp portion 103. When thevoltage of the ramp 104 is at a level below that corresponding to a onetrack reset, no reset pulses are generated and the transducing head 30will merely follow the next track rather than being reset to rescan thesame track. It should also be appreciated that the reset pulses are onlyproduced during the drop out interval and are inhibited when thetransducing head 30 is scanning a track and reproducing active videoinformation. In other words, the level of the voltage of the ramp 104 isdetected at the decision point 108 of the ramp 104 just before the dropout interval 102 and, if it is found to be within reset range, anappropriate reset pulse will be generated and applied during the dropout interval for deflecting the movable element 32 the required amountin the direction opposite that it was previously deflected by the rampportion 104 of the head deflection voltage waveform.

To more readily visualize the function of the forward and reversedirection reset pulses, reference is made to FIG. 6, which illustrates apath 110 shown by phantom lines followed by the scanning head 30relative to the tape 36 during a stop mode of operation. As seentherein, the head starts its scan of the tape 36 at the beginning oftrack F and cuts across the track to the end of track E during a singlerevolution. This occurs if the tape 36 is not moving and the transducinghead 30 is not deflected. It should therefore be appreciated, that ifthe automatic head tracking circuitry is operative to maintain thetransducing head 30 so as to follow track F, the head will gradually bedeflected in the reverse direction by a ramp portion of the headdeflection waveform, i.e., in the direction opposite the arrow 38, andif it were not deflected at the end of the track F, it would be in aposition to begin playing the track G. To rescan track F, it isnecessary to apply a reset pulse that will move the head 30 in theforward direction, i.e., in the direction of the arrow 38 so as to havethe head in position to begin reproducing the beginning of track F.Thus, the reverse and forward terms in FIGS. 7b-7g are in the context ofreverse and forward directions of tape movement and the movement of thehead is referenced to these same directions.

In accordance with the record/reproduce apparatus described herein, thecircuitry for generating the reset pulses is operable to selectivelygenerate the reset pulses, depending upon the mode of operation of theapparatus. Thus, referring to FIGS. 7d, 7e, 7f, and 7g, it is seen thatreset pulses will not be produced when the head 30 is deflected in theforward direction by an amount less than a selected distance dependingupon the operating mode and a single reset pulse will be produced toreset the head 30 in the reverse direction when the head is deflected inthe forward direction by an amount greater than the distance separatingadjacent tracks. This appears in all of the diagrams shown in FIGS. 7d,7e, 7f, and 7g. The reverse direction reset pulses will regularly occurwhen the tape is moving at a speed between normal speed and twice normalspeed.

When the record/reproduce apparatus is operating in the slow/still mode,it is desired that reset pulses be generated in the same manner as wasperformed by the apparatus disclosed in the aforementioned Hathaway etal. application. Accordingly, the diagram shown in FIG. 7d illustratesthe operation circuitry of the apparatus when it is operating in theslow/still mode; and it is seen that its characteristics for headdeflections in the reverse directions are similar to those shown in thediagram of FIG. 7c. Typically, when operating in the slow/still mode, ifthe waveform 104 at the end of a track scan corresponds to a headdeflection from zero to just greater than one track center-to-trackcenter spacing in the reverse direction, then a track reset will occurwhich will move the transducing head 30 in the forward direction adistance equal to the separation of adjacent track centers. The headdeflection waveform 104 of FIG. 7d depicts the operating conditionwhereby the movable element 32 is deflected between its zero deflectioncondition and a deflection condition just greater than one trackcenter-to-track center spacing in the its forward direction.

However, as can be seen from the head deflection waveforms 104, 106 and104', 106' shown in FIG. 7e and 113 shown in FIG. 7d, the average levelof the head deflection waveform, hence, average position of the movableelement 32, can vary for the same head tracking condition. For theoperating modes illustrated by FIGS. 7d, 7e, 7f and 7g, the headposition waveform can be anywhere within a range corresponding to 1track deflection in the forward direction and 1 track deflection in thereverse direction for any instantaneous head tracking condition andprecise head tracking will be maintained. A different position withinthe range only has the effect of altering the average position aboutwhich the movable element 32 is deflected.

FIG. 7d includes a head deflection waveform 104, 106 shown by phantomlines for a slow motion speed of 1/2 normal speed. As shown therein,this slow motion operation results in the movable head 30 being resetafter every other one of its rotations to rescan every other track,hence, field a second time. Between consecutive resets of the movablehead 30, the head is deflected to account for the different path anglethe head would otherwise follow along the tape 36 and allowed to scantwo adjacent tracks during successive rotations of the head 30.

FIG. 7d also includes a head deflection waveform 113, 115 shown byphantom lines for a stop motion or still image operation wherein twoadjacent tracks are consecutively scanned to reproduce two consecutivetelevision flelds before the movable head 30 is reset or repositioned torescan the tracks. This is in contrast to the stop motion operationpreviously described with reference to FIG. 7c, wherein the movable head30 is controlled to scan a single track repetitively to reproduce asingle television field for the generation of the desired still imagedisplays. As will be described in detail hereinbelow with reference toFIGS. 10a, 10b, 10c and 10d, the record/reproduce apparatus includes atransducing head tracking servo that employs circuitry for detectingwhen the movable head 30 must be repositioned or reset to rescanpreviously scanned tracks and applying a reset signal to the movableelement 32 at the proper time. This detection and resetting circuitry isarranged to selectively permit still image reproduction from a singlerepetitively reproduced field, a repetitively reproduced sequence of twofields, i.e., a monochrome frame, or a repetitively reproduced sequenceof four fields, i.e., a color frame. The selective monochrome frame orcolor frame still image reproduction is achieved by means that prohibitsthe application of the head repositioning reset signal that normally isapplied at the end of the scan of each track when in the still modeuntil the desired sequence of fields has been reproduced and by meansthat applies the appropriate amplitude reset pulse to reposition thehead 30 to the track containing the first field of the sequences uponeach completion of the sequence.

The head positioning waveform 113, 115 shown in FIG. 7d illustrates themanner in which the movable head 30 is deflected to repetitivelyreproduce a sequence of two fields recorded in adjacent tracks so thatmonochrome frame still image displays can be generated. Generating stillimage displays from a monochrome frame composed of two consecutivelyreproduced fields has the advantages over the use of a single field ofincreased vertical resolution of the image (525 line resolution insteadof 2621/2 line resolution) and of avoiding the necessity of introducinga 1/2 line delay in alternate reproductions of a single field.Generating still image displays from a color frame composed of fourconsecutively reproduced fields has the further advantage of providingthe entire color information content of the displayed image and ofavoiding the necessity of separating the luminance and chrominancecomponents of a composite video signal so that the chrominance componentcan be inverted to provide the proper color subcarrier phase whenforming a still image color display from a single field or a monochromeframe.

The aforedescribed operation of the transducing head tracking servo forgenerating a still color image display from a sequence of fieldscontaining the entire color code sequence is described as arranged togenerate the still displays from an NTSC standard color televisionsignal, which requires four consecutive fields to color encode thesignal. In the PAL and SECAM standards, color frames are composed of 8and 4 fields, respectively. As described hereinbelow, the head trackingservo can be arranged to reproduce a color frame in each of thesestandards in the still frame mode. For PAL standard color televisionsignals, the head positioning reset signal is inhibited to permit thereproduction of 8 consecutive fields before a head positioning resetsignal is provided to effect the repositioning of the head 30 to rescanthe 8 consecutive fields. While SECAM standard color television signalshave a 12 field color frame sequence, the nature of SECAM signalsenables satisfactory color displays to be generated from the repetitivereproduction of 4 consecutive fields. Therefore, the head positioningreset signal is inhibited to permit the reproduction of 4 consecutiveSECAM standard fields before a head positioning reset signal is providedto effect the repositioning of the head 30 to rescan the 4 consecutivefields.

It should be appreciated that if relative motion is present in theimages represented by two or more television fields used to generatemonochrome frame or color frame still images, jitter will be present inthe repetitively displayed monochrome or color frame. If the jitter isobjectionable, the monochrome or color frame display can artificially begenerated from a single field or only those fields without relativemotion.

Although readily apparent from the above description of the improvedrecord/reproduce apparatus, it should be emphasized that, when in themonochrome frame or color frame still image mode, the tap 36 istypically stopped and the head 30 is continuously deflected, forexample, as depicted by the ramp portion 113 of the head deflectionwaveform shown in FIG. 7d, between the applications of appropriatelytimed consecutive head reset signals, such as, for example, reset step115 in FIG. 7d. With respect to the particular embodiment of theautomatic tracking circuitry shown in FIGS. 10a and 10b, in color framestill image modes, the variable reference threshold circuitry 126 (FIG.3) employed in conjunction with associated latches and gates to generatethe appropriate amplitude head resetting signal is modified to includeadditional parallel latches and gates as shown in and describedhereinbelow with reference to FIG. 10d. Also, as shown in FIG. 10c, andwill be described hereinafter, the ambiguous head track lock circuitryincludes means to properly time its operation so that artificial headresetting signals are properly provided in accordance with theparticular still frame mode.

When the apparatus is switched from the slow/still motion mode ofoperation to normal speed mode of operation, the tape transport servosystem accelerates the tape 36 up to about 95% of normal speed. Duringthe tape acceleration interval, which lasts about 0.5 sec. when the tape36 is accelerated from stop, the variable reference threshold circuitry126 establishes the same head reset reference threshold levels as itdoes for slow/still operating modes. Upon reaching 95% of normal speed,the automatic head tracking servo circuitry switches to have thecharacteristics shown in the diagram FIG. 7e, which is different thanthe slow/still characteristic shown in FIG. 7d in that a reset pulse isproduced for head deflections in the reverse direction in an amount lessthan one-half the spacing between adjacent track centers. However, a onetrack reset pulse will continue to be produced to move the head 30 inthe forward direction whenever the head is deflected in reversedirection by an amount in the range of one-half to just greater than thedistance between adjacent track centers. It is during this time when thetape 36 is being transported at the 95% normal speed, that the initialcolor frame determination is made. During this initial determinationstage, it is desired that the forward reset pulses be provided onlywhenever the movable head 30 is deflected in the reverse direction anamount between one-half and just greater than the distance betweenadjacent track centers so that the head positioned correction waveformwill remain more closely centered about the zero voltage level, ratherthan at an average negative value as could be the case with respect toFIG. 7d. By not resetting the head 30 when it is deflected in thereverse direction by an amount less than one-half the distanceseparating adjacent tracks, the average value of the head deflectionwaveform will more closely approach that shown in FIG. 7b, where it isgenerally centered around the zero head deflection mark. Once theinitial color framing determination operation is complete and providedthat the phase of the control track signals is within a predetermined"window" when compared to a reference signal, as will be hereinafterdescribed, the tape transport servo system switches from the 95% normalspeed to 100% or normal speed. The tape 36 is quickly accelerated to100% of normal speed and the automatic tracking circuitry is thenswitched to the normal speed mode which has the characteristicsillustrated in FIG. 7f. However, before initiating normal reproductionoperations in the normal speed mode, the reproduced video signal isexamined to determine whether the initial monochrome and color framedetermination has been correctly made. Because the aforementionedmachine-to-machine tolerance variations in professional quality videorecord/reproduce apparatus typically do not vary outside of a tolerancerange that would produce more than a plus or minus one (1) track headpositioning error when monochrome and color framing relative to therecorded control track signal, the apparatus herein described can takeadvantage of the information content of the reproduced video signal's Hsync to V sync phase relationship, i.e., monochrome frame information toverify the correctness of the initial monochrome and color framing. Aswill be described in further detail hereinbelow, the reproduced videosignal's H sync to V sync phase relationship is compared to theequivalent phase condition of the studio reference. If the monochromeframe of the reproduced video signal differs from that of the studioreference, the automatic tracking circuitry responds to a field matchsignal generator 95 (FIG. 2) to deflect the movable element 32 adistance equal to that separating adjacent track centers and in theproper direction to achieve color framing. FIG. 7f includes a headdeflection waveform 106, 109 shown by phantom lines for a normal speedmode of operation, including a forward reset portion 106 representing atypical one track deflection of the head 30 for color framing purposesfollowed by a typical head position correction waveform 109 occurringduring normal speed mode operations. Furthermore, as shown in FIG. 7f,the normal speed dynamic range of the automatic tracking circuitry isshown to extend from a head deflection in the forward direction justgreater than the distance separating adjacent track centers to a headdeflection in the reverse direction of a corresponding amount, whichmeans that no reset will occur if the instantaneous voltage level justbefore the drop out interval 102 is within this dynamic range. Thesingle track reset pulses (in both directions) are provided to centerthe transducing head 30 if an external disturbance or the like causesthe movable element 32 carrying the transducing head 32 to be outside ofits normal operating range.

In the two times normal speed mode, the tape 36 is transported past thescanning head location at a rate that is two times that for the normalspeed operating mode. Consequently, as a track is being scanned by thehead 30 during this mode, the track is advanced a distance in theforward direction beyond the scanning head location corresponding to thedistance separating adjacent track centers. Therefore, to maintainhead-to-track registration, the scanning head 30 must be deflected inthe forward direction a corresponding distance during the scan of atrack. Two times normal speed motion is achieved by reproducing everyother recorded field at the normal field rate for video signals, i.e.,60 Hz. By resetting the position of the scanning head 30 in the reversedirection at the conclusion of the scan of a track a distancecorresponding to the distance separating adjacent tracks, the scanninghead 30 skips the adjacent downstream track that it would normallyfollow if not reset, which contains the next field of the recordedsequence of video fields, and instead is positioned to reproduce thefield recorded in the track that is located two recorded track positionsfrom the track whose scan has just been completed. FIG. 7g illustratesthe head deflection waveform generated by the circuitry 90 when the tapetransport servo system is controlled to transport the tape at two timesnormal speed. As can be appreciated from the illustrated waveform, whenthe tape 36 is transported at twice normal speed, the movable head 30 isdeflected in the forward direction an amount exceeding the distanceseparating adjacent track centers. When the deflection exceeds thatamount, a one (1) track reverse reset pulse is produced to position thathead 30 over a track located two recorded track positions from the trackwhose scan has just been completed.

The operational characteristics shown in FIG. 7d, 7e, 7f, and 7g arecarried out by the circuitry 90 shown in the block diagram of FIG. 3.The mode control line 92 is connected to logic circuitry indicatedgenerally at 111 and has lines 112, 114, 116 and 118 extending torespective switches 120, 122, 124 and a variable reference or thresholdproducing circuit 126. The error detector output signal from thesynchronous detector 78 (FIG. 1) is applied via line 80 to the switches120 and 122, only one of which can be closed at one time by operation ofthe logic circuitry 111. The switch 120 is connected via line 128,resistor 130 and line 132 to the negative input of an integrator 134,while the switch 122 is connected via line 136, resistor 138 and line132 to the same integrator input. The values of the resistors 130 and138 are different and effectively change the loop gain or compensationof the error signal on line 80 as applied to the input line 132 of theintegrator 134 according to which one of the switches 120 or 122 isclosed. When the apparatus is operating in the slow/still mode, switch120 is closed and switch 122 is open so that the gain of the head trackpositioning servo system is increased so it can react faster, sincethere is more movement required of the movable element 32 carrying thetransducing head 30 during the slow/still mode of operation than in mostother modes. When the apparatus is placed in normal speed mode, switch122 is closed and switch 120 is open so that the gain is reduced, lessmovement for correction being required in this mode because thetransducing head 30 will normally closely follow the track. When theapparatus is in its slow/still mode of operation, switch 124 is alsoclosed to connect a DC voltage centering network 139 for the integrator.During slow motion modes of operation below one-half normal speed, thereis a need for the centering network around the integrator 134 to preventthe integrator from swinging too far out of its normal operating rangeand, thereby, require excess time for servo acquisition after theapparatus is turned on. During the normal speed mode, the network 139 isunnecessary and therefore switch 124 only brings it into operationduring the slow/still mode of operation. Furthermore, when reproducedvideo is initialy detected during an operating mode signified by a highlogic RF PR signal level on input line 123 (FIG. 10a), the logic circuit111 functions to close switch 124, to facilitate rapid servo locking.

When the error signal is applied to the input line 132 of the integrator134, the error signal causes the transducing head 30 to be adjusted soas to follow the track regardless of the speed of tape transport,provided it is within the limits of deflection of the movable element32. The integrator 134 provides a ramp signal that has a slope which isdetermined by the speed of transport of the tape 36 and an average DCvalue that is determined by the DC or low frequency error signal that isderived from the head tracking servo circuitry. Thus, the servo errormodulates the average level of the ramp as the transducing head positionerror changes and the output of the integrator appears on line 66, whichextends to the summing circuit 64 shown in FIG. 1. The reset pulses aresummed at the input line 132 of the integrator 134, with the resetpulses being derived from the processed drum once around tach andselectively passed by AND gates 140, 142 and 144. The processed oncearound tach is derived from a tach pulse generated by a tachometer (notshown) operatively associated with the rotating drum 22, one tach pulsebeing provided for each revolution of the rotating drum, hence, thescanning head 30. Conventional tachometer processing circuitry providesthe pulse at the desired system time and of selected width. The AND gate140 has its output connected to line 132 via a resistor 146 and AND gate142 has its output connected to line 132 via a resistor 148 and theoutput of AND gate 144 is connected to an inverter 150 which in turn isconnected to line 132 via a resistor 152. If either of the AND gates 140or 142 are activated, then a predetermined current pulse whose amplitudeis determined by resistors 146, 148 and 152 will appear on line 132 andbe applied to the integrator 134 for the purpose of resetting thevoltage level at the output thereof. The actuation of either of the ANDgates 140 or 142 will produce a reset step in the output of theintegrator 134 of predetermined value that will correspond to the properamplitude reset step required to deflect the movable element 32 adistance in the forward direction corresponding to the center to centerdistance between adjacent tracks, i.e., a one track position deflectiondistance. If the AND gate 144 is actuated, then by virtue of theinverter 150, an opposite polarity reset pulse is produced on line 132,as compared to the polarity of the pulse from the AND gates 140 and 142,and which opposite polarity effectively causes a reset of the movableelement 32 in the reverse direction as is desired. If both of the ANDgates 140 and 142 are activated simultaneously, for example, as occursduring the 95% normal speed mode when the head 30 is deflected in thereverse direction a distance greater than that corresponding to thetrack-to-track separation, a twice amplitude current pulse will appearon line 132 and be applied to the integrator 134 for the purpose ofresetting the voltage level at the integrator's output, hence, theposition of the movable head 30, the equivalent of two track positionsin the forward direction.

The output line 66 of the integrator 134 is coupled to one input of eachof three level detectors 156, 158 and 160, each of which effectivelymonitors the instantaneous voltage on line 66 to determine if resetpulses are to be generated. The level detector 156 has its other inputcoupled to line 162, which is provided with a constant threshold voltagethat corresponds to the level for producing the one track forward resetpulse shown in FIGS. 7d, 7e and 7f. Thus, if the instantaneous voltagelevel on line 66 exceeds the value of the threshold voltage on line 162,i.e., the instantaneous level is above the one track reverse thresholdvoltage, then a forward reset pulse will be generated. The leveldetector 160 has its other input coupled to line 187, which is providedwith a constant threshold voltage that corresponds to the level forproducing the one track reverse reset pulse shown in FIG. 7g. If theinstantaneous voltage level on line 66 is less than the value of thethreshold voltage on line 187, i.e., the instantaneous level is belowthe one track forward threshold voltage, a reverse reset pulse will begenerated. The level detector 158 has its other input coupled to thevariable reference 126 and, as will be explained further hereinafter, itreceives one of alternative reference level signals, the selectedalternative being dependent upon the operating mode of therecord/reproduce apparatus. In the embodiment of the apparatus shown byFIGS. 10 and 11, the variable reference 126 establishes thresholdvoltage levels used to control the generation of forward head positionreset pulses in operating modes below normal speed.

To generate the reset pulses, each of the level detectors 156, 158 and160 have respective output lines 164, 166 and 168 which are respectivelyconnected to the D input of latches 170, 172, and 174. The Q outputs ofthe respective latches are connected via lines 176, 178 and 180 to theAND gates 140, 142 and 144. A line 182 is connected to the clock inputs,C, of the latches 170, 172 and 174 and to a pulse and clock generatorcircuit 184. The generator curcuit 184 also has an output line 186connected to a second input of the respective AND gates 140, 142 and144. A pulse derived from the processed once around tach is used by thecircuitry 90 to trigger the pulse and clock generator circuitry 184 andto clock the latches 170, 172 and 174. In one embodiment of theapparatus described herein, the tachometer processing circuit generatesthe processed drum tach pulse about 16 msec. after the occurrence of theonce around drum tachometer pulse. The once around drum tach pulseoccurs at the beginning of the drop out interval 102 (FIG. 7a). The 16msec. delayed processed drum tach pulse is timed to occur at thefollowing track reset decision time, identified in FIGS. 7b-e and 7f bythe reference number 108. It is this processed drum tach pulse thatclocks the latches 170, 172 and 174 to enable them to latch thecondition of the outputs of the level detectors 156, 158 and 160,thereby, determining whether a step reset of the movable head 30 isrequired. As will be described in further detail hereinbelow, the actualreset pulse is generated by the pulse and clock generator 184 from theprocessed drum tach pulse, but delayed about 0.67 msec so that any stepresetting of the movable head 30 occurs during a drop out interval 102.During operation, if the instantaneous voltage on line 66 at theoccurrence of the processed once around tach pulse on line 182 exceedsthe particular value of the threshold voltage applied at the input ofthe respective level detectors, the output line associated with each Qoutput of the level detectors whose threshold voltage is exceeded willbe latched to a high logic level by the clocking action of the procesedonce around tach signal on line 182. For example, if the instantaneousvoltage on line 66 exceeds a level corresponding to a head deflection inthe reverse direction in excess of the distance represented by thereference threshold voltage provided by the variable reference generator126 (i.e., any reverse deflection of the movable element 30 when in theslow/still operating mode and a reverse deflection, in excess ofone-half the distance separating adjacent track centers when in thenormal 95% normal speed operating mode), latch 172 is conditioned toenable the associated AND gate 142 to provide a single 1 track resetpulse for effecting a forward 1 track step deflection of the movablehead 30. On the other hand, if the instantaneous voltage on line 66exceeds a level corresponding to a head deflection in the reversedirection in excess of the distance separating adjacent track centers,both latches 170 and 172 are conditioned to enable their respectiveassociated AND gates 140 and 142 to provide 1 track reset pulses, whichare summed at the input line 132 of the integrator 134, therebyeffecting a forward 2 track step deflection of the movable head 30. Inthe event the instantaneous voltage on line 66 exceeds a levelcorresponding to a head deflection in the forward direction in excess ofthe distance separating adjacent track centers, latch 174 is conditionedto enable the associated AND gate 144 and following inverter 150 toprovide a 1 track reset pulse for effecting a reverse direction onetrack step deflection of the movable head 30.

The line 118 from the logic 111 controls the variable referencecircuitry 126 to provide a threshold voltage on line 196 that variesbetween three levels so as to accomplish selective resetting of theposition of the movable head 30, depending upon the operating mode ofthe apparatus, as shown in FIGS. 7d, 7e, 7f and 7g. As describedhereinbefore, when the apparatus is operating in the slow/still mode,the circuitry 126 provides a threshold voltage such that a forward headposition reset occurs when the voltage level on line 66 exceeds a levelcorresponding to any head deflection in the reverse direction at theoccurrence of a processed drum tach signal on line 182. When theapparatus is switched from the slow/still mode to the 95% of normalspeed mode, the variable reference circuitry 126 applies a differentthreshold to the level detector 158 so that a 1 track forward resetpulse is produced only when the voltage on line 66 at the occurrence ofa processed drum tach pulse exceeds a level corresponding to any headdeflection in the reverse direction in excess of one-half the distanceseparating adjacent track centers. Similarly, when the apparatus isswitched to the normal speed mode, the variable reference circuitry 126supplies a voltage level to the level detector 158 that disables it sothat a pulse cannot be passed by its associated AND gate 142 regardlessof the instantaneous level on line 66. The one forward reset pulse thatis generated in the normal speed mode when the instantaneous voltage online 66 exceeds the level corresponding to a head deflection in thereverse direction exceeding a distance of about 1.1 times the separationof adjacent track centers, is produced by the operation of the leveldetector 156. As described hereinbefore, the threshold level forinitiating a forward reset step of the movable element 32 is increasedin steps from a level corresponding to no head deflection in the reversedirection to a level corresponding to a head deflection in excess of thedistance separating adjacent track centers as the video record/reproduceapparatus operating mode is changed, for example, from still motion tonormal speed forward motion. This keeps the head positioning waveformgenerated by the integrator 134 at an average level near zero deflectionso when the tape 36 is accelerated to 100% normal speed, the video head30 will be positioned to scan the right track for proper monochromeframe and color frame conditions relative to the studio reference.

With respect to the diagram shown in FIGS. 7d and 7e, where a two trackforward head positioning reset pulse indication is shown to be producedwhen the voltage on line 66 exceeds that corresponding to a reverse headdeflection in excess of the distance separating adjacent track centers,this is accomplished by both level detectors 156 and 158 going highwhich produces a double amplitude forward reset pulse as previouslyexplained. Both level detectors 156 and 158 cause the enabling of theassociated AND gates 140 and 142, respectively, because whenever areverse head deflection exceeds the distance separating adjacent trackcenters, the voltage on line 66 will exceed both threshold levelsestablished for the level detectors during the operating modesillustrated by FIGS. 7d and 7e.

With respect to the two times normal speed mode illustrated by FIG. 7g,the level detector 168 functions to cause its associated AND gate 144and following inverter 150 to deliver an opposite polarity 1 trackreverse reset pulse to the integrator 134 to effect the resetting of themovable head 30 because, at the end of the head scan of each track, thevoltage level on line 66 exceeds the threshold level established for thelevel detector on line 187.

With respect to the control of the transport of the tape 36 around thetape guide drums 22, 24 during recording and reproducing operations,reference is made to FIG. 8, which is an electrical block diagram ofcircuitry of a tape transport servo system that can be used to controlthe transport of the tape. As previously mentioned, when the apparatusis switched from the slow/still mode of operation to the normal speedmode, the tape transport servo circuitry is made to follow the speedprofile shown in FIG. 9. In video tape record/reproduce apparatus, thetape 36 is conventionally transported by a capstan 200, which is drivenby a motor 202 through a shaft 204. A capstan tachometer 206 is operablyconnected to the shaft 204 to provide signals indicative of the rotationof the shaft 204 and the signals appear on line 208 which is coupled toa frequency discriminator 210, to variable slow motion control circuitry240 and to a phase comparator 212.

The frequency discriminator 210 provides a signal indicative of thevelocity at which the capstan 200 is driven. Its output is connected toa summing circuit 214 via line 216 so that the capstan velocity relatedsignal provided by the frequency discriminator 210 is subtracted fromthe reference velocity drive signal provided by a velocity referencecircuit 250 for correcting the velocity drive signal provided to thecapstan 200. The output of the summing circuit 214 is connected via aswitch means 226 and line 218 to a motor drive amplifier 220 that drivesthe motor 202 via line 222. The circuitry is controlled by an operatorapplying, through the operation of appropriate control devices, modecommands to logic circuitry 224, which in turn provides commands to theautomatic head tracking circuitry previously discussed as well as to thetwo position switch means 226 having a movable contact means 228 thatcan switch between positions 1 as shown or position 2. The commands fromthe logic circuitry 224 are coupled via control lines 230, these linesalso being coupled to control a switch means 232, which has a movablecontact means 234 that is capable of being positioned in one of threepositions. When the apparatus is operated in the slow/still mode, toprovide slow motion reproductions of the recorded video signalsrequiring very low tape transport speeds, typically, less than 1/5normal speed, a variable slow motion control 240, includng a tape speedcontrol 240' potentiometer, is adapted to apply a pulse drive signal tothe motor drive amplifier 220 via a line 242, contact means 228 ofswitching means 226 (in position 1), line 218. When in this mode, switchmeans 232 is in position 1 and drive of the capstan motor 202 providedby the motor drive amplifier 220 is controlled during the very low tapespeeds solely by the drive signal generated by the variable slow motioncontrol 240. The variable slow motion control 240 provides the pulsedrive signal to drive the capstan motor 202 until the velocity of thetape 36 reaches about 1/5 normal speed. At this tape speed, velocitycontrol of the tape drive is switched over to the velocity referencecircuit 250, which responds to the tape speed control potentiometer tochange the drive signals to motor 202 and selectively vary the speed ofthe tape 36. The apparatus described herein employs the variable slowmotion control circuitry described in the aforementioned Mauchapplication, Ser. No. 874,739.

To switch the velocity control drive from the variable slow motioncontrol circuit 240 to the velocity reference circuit 250 at theaforementioned cross-over velocity range, the logic circuitry 224operates the switch means 226 so that the movable contact means 228 iseventually placed in position 2 and triggers a velocity referencecircuit 250 via a command placed on line 252 extending from the logiccircuitry 224. The velocity reference circuit 250 responds to thecommand placed on line 252 to generate a voltage level in accordancewith the position of the operator controlled potentiometer 240'. Thevoltage level is coupled by line 254, summing circuit 214, contact means228 of switching means 226 (in position 2) and line 218 to the motordrive amplifier 220. For the acceleration mode, the logic circuitry 224provides a command on line 252 that triggers the velocity referencecircuit 250 to provide a voltage ramp of selected rate and duration, toaccelerate the tape 36 to 95% normal speed within an interval of 0.5sec. When the record/reproduce apparatus is placed in the accelerationmode, the logic circuit 224 issues a command over a control line 230 tocause the movable contact means 228 of the switch means 226 to be placedin position 2 so that the voltage ramp signal is coupled via line 218 tothe motor drive amplifier 220 to effect acceleration of the tape 36.

The velocity reference circuit 250 provides the capstan drive velocityservo reference signal for controlled slow motion operating speeds abovethe cross over tape velocity of about 1/5 normal speed and foraccelerating the tape 36 to 95% normal speed when the apparatus isoperated to enter a normal speed reproduce mode. During these operatingmode conditions, the applied ramp or voltage level velocity servoreference drive signal causes the motor to transport the tape 36 atabout the desired speed. The line 208 from the tachometer 206, togetherwith the frequency discriminator 210, line 216, summing circuit 214,contact means 228 and line 218 provide a velocity lock mode ofoperation, which forces the capstan to follow the velocity servoreference drive signal provided by the velocity reference circuit 250.In this regard, it should be noted that the switch means 232 has themovable contact means 234 in position 1 during the velocity lock mode ofoperation.

When accelerating the transport of the tape 36 to enter the 95% normalspeed mode, the capstan 200 accelerates the tape 36 to the 95% normalspeed level and, upon reaching that speed, switch means 232 is switchedby the operation logic circuitry 224 so that the movable contact means234 is in position 2. This places the capstan velocity servo in acapstan tach phase lock mode of operation. In this mode, the phasecomparator 212 compares the phase of the capstan tach signal on line 208with a tach related servo reference signal, which is coupled to line 258by a variable divider 260. The variable divider 260 is controlled by acontrol signal placed on the control line 262 by the logic circuitry 224together with clock signals on line 264 supplied by clock circuitry 266.The clock signals are in the form of a 64H reference signal provided bya conventional video reference source commonly found in videorecord/reproduce apparatus. The control signal line 262 sets thevariable divider 260 so that it provides a divided clock signal to thephase comparator 212 that maintains the speed of the tape 36 at the 95%normal speed until the initial color frame determination has beencompleted, as generally described hereinbefore and will be described infurther detail hereinbelow.

When the initial color frame determination has been completed, it isthen desired to switch from the 95% normal speed mode to the normalspeed mode, which requires the tape 36 to be accelerated up to the 100%normal speed. However, before the final acceleration is performed, it isdesirable, in addition to making the initial color frame determination,to continue the 5% slip or slewing until the phase of the off tapecontrol track 94 is within a predetermined window when compared with thecontrol track reference signal, i.e., within about plus or minus tenpercent (10%) of the control track servo reference signal. This isdesirable in order to insure that when the control of the capstan 200 isswitched to the control track phase lock mode from the capstan tachphase lock mode that there be a minimum tape velocity disturbanceintroduced to the tape transport servo. If, for example, the controltrack loop was enabled when the control track was not within the phasewindow with respect to the control track servo reference, an undesirabletape speed transition may occur due to the tape transport servo looptrying to rephase the transport of the tape 36 and the transition may bedrastic enough that the initial color frame condition may be lost.

A control track head 267 of the video record/reproduce apparatus detectsthe recorded control track 94 and couples it to line 268 extending tothe input of the color frame detector 280 and control track phasecomparator 270. The phase comparator 270 serves to compare the phase ofthe reproduced control track signal on line 268 with a 30 Hz controltrack servo reference signal on line 272 from the system clock circuitry266. The phase comparator 270 is a typical circuit employed in thecontrol track servo loop of helical scan video tape recorders, such asthe VPR-1 video production recorder identified herein. Before the tape36 is accelerated to 100% normal speed and the apparatus is switchedfrom the capstan tach phase lock mode to the control track phase lockmode, the initial color frame determination is made by the color framedetect circuitry 280 typically included in helical scan video recorders,such as the above-identified VPR-1 video production recorder. The colorframe detector 280 compares the 15 Hz color frame component of therecorded control track 94 reproduced on line 268 by the control trackhead 267 with a color frame reference signal provided on line 282 by thesystem clock circuitry 266. When the signals received by the color framedetector 280 indicate an initial color frame condition, an output signalis provided on line 284 to the logic circuitry 224. Before finalacceleration of the tape 36 to 100% normal speed, the output of thephase comparator 270 is coupled by line 274 to the input of a typicalcontrol track error window detector 276, such as also included in thecontrol track servo loop of VPR-1 type helical scan recorders. Thedetector 276 is further connected via its output line 278 to the logiccircuitry 224. If the control track error signal provided by the phasecomparator 270 is within the error window established by the windowdetector 276, an enabling signal is issued over line 278 to the logiccircuitry 224.

The logic circuitry 224 responds to the afore-described inputs receivedfrom the color frame detector 280 and the control track error windowdetector 276 by activating the control line 262 to set the variabledivider 260 so that capstan tach phase comparator 212 receives a servoreference input corresponding to the tape 36 being transported at 100%normal speed. Following an interval of about 0.5 sec., during which thecorrectness of the initial color framing is verified as generallydescribed hereinbefore and an appropriate one track head positioningcorrection is made if the initial color framing was in error, themovable contact means 234 of the switch means 232 is placed in position3. This places the capstan 200 under servo control of the control trackphase comparator 270 by coupling the output line 274 of the comparatorto the summing circuit 214 via switch contact means 234 and line 244.The capstan motor 202 is now servo controlled by the recorded controltrack signal via the motor drive amplifier 202 and its input line 218extending from the summing circuit 214 and the record/reproduceapparatus ready for synchronous reproduction of the recorded signals.

Specific circuitry that can be used to carry out the operation of theblock diagrams shown in FIGS. 3 and 8 are illustrated in FIGS. 10a and10b as well as FIGS. 11a, 11b and 11c. The specific circuitry shown inFIGS. 10a and 10b illustrate the automatic tracking circuitry shown inthe block diagram of FIG. 3, together with portions of the circuitryshown by the block diagram of FIG. 1. The circuitry shown in FIGS. 10aand 10b, to the extent that it includes circuitry represented by theprior art block diagram of FIG. 1, is contained in and is also describedin catalogs illustrating the detailed construction of the prior artapparatus. In this regard, reference is made to catalogs of the VPR-1Video Production Recorder, catalog Nos. 1809248-01 dated January, 1977and 1809276-01 dated February, 1977 prepared by the Audio-Video SystemsDivision of Ampex Corporation, Redwood City, California, which catalogsare hereby incorporated by reference herein. In this regard, thecircuitry shown in FIGS. 11a, 11b and 11c also incorporate circuitrythat exists and is illustrated in the above-referenced catalogs. Theoperation of the circuitry shown in FIGS. 10a, 10b, 11a, 11b and 11cwill not be described in detail since they generally carry out theoperation previously described with respect to the block diagrams ofFIGS. 3 and 8. Moreover, the schematic diagrams contain circuitry whoseoperation is not directed to the specific invention described herein andperform functions that can best be understood from the overall operationof the video production recorder, the complete electrical schematics ofwhich are shown in the aforementioned catalogs. However, to the extentthat the operation of the block diagrams can be directly correlated tothe specific schematic circuitry, reference numbers will be includedthereon and certain operations will be hereinafter described.

Turning to the electrical schematic diagram of FIGS. 10a and 10b, the RFsignal from the equalizer circuitry 74 is applied via line 75 to anautomatically calibrated RF envelope detector circuit 76 in accordancewith the present invention, which further includes an automaticreference level setting feedback loop 299. Envelope detector circuit 76includes a variable gain amplifier 301 coupled via output pin 8 to anenvelope detector 303 (pin 7) which detects the amplitude of the RFenvelope as modulated by the dither signal. Amplifier 301 and detector303 herein are integrated circuits having a standard industrydesignation of MC 1350 and MC 1330 respectively, wherein correspondingpin number connections are identified in the drawings for referencethereto. As previously mentioned, the amplitude and polarity of the RFenvelope modulation are indicative of the amount and directionrespectively of lateral head displacement from track center. Therefore,it is necessary that the envelope detector circuit 76 provide a constantdemodulation gain for proper head tracking servo operation. However,detector integrated circuits such as detector 303, exhibit varyingsensitivities and DC offset characteristics from chip-to-chip, whichinherently causes corresponding variations, and thus inaccuratemeasurement of the detected amplitudes. Likewise, different tapeformulations, different heads, head and/or tape wear, variations inhead-to-tape contact, etc., cause differences in recorded RF levelsbetween tapes, which also results in inconsistent envelope detectorcircuit output signals. The feedback loop 299 thus provides means forautomatically compensating for differences between IC componentcharacteristics, tape RF level differences, etc., to provide a constantdetector circuit 76 output under all conditions.

To this end, a capacitor 305 is coupled between the output of thedetector 303 (pin 4) and a junction of switches 307, 309. The otherterminals of switches 309, 307 are respectively coupled to a 5 voltsource and to the negative input of a differential amplifier 311. Thelatter's positive input is selectively referenced to a +2 volt level viaa resistive divider network 281 and +5 volt source. An RC network 313and a diode 315 are coupled across the amplifier 311 negative input (pin2) and the output (pin 1), with the output coupled in turn to thecontrol input (pin 5) of the variable gain amplifier 301 as well as to a12 volt source via a zener diode 317. The switches 307, 309 arecontrolled via inverters coupled to the true and not true outputs (pins13 and 4) respectively of a one-shot multivibrator 319. The one-shotgenerates a pulse which approximately matches the drop out interval 102(FIG. 7a) of the RF envelope, and is clocked via the drum tachometersignal received from the drum tachometer processing circuitry over line321, to alternately close switch 307 during the interval of thereproduced RF envelope 100 and switch 309 during the drop out interval102 (FIG. 7a).

During each drop out interval, i.e., once for each transducing headrevolution, the RF envelope amplitude is zero, i.e., there is 100%modulation of the envelope, whereby during each closure of the switch309, a reference level change of +5 volts is set between capacitor 305and ground. When switch 307 is closed during the reproduction of the RFenvelope, the feedback loop 299 is referenced to +2 volts, thus forcingthe reference level setting feedback loop 299 to automatically servo a+3 volt change at the output of detector 303 and thereby provide aconstant demodulator gain from the envelope detector circuit 76,regardless of any variations in the tape RF levels, componentcharacteristics, etc. The +3 volt change is equivalent to the averageamplitude of the RF envelope without amplitude modulation at the outputof the envelope detector circuit 76 with the desired average amplitudefor an unmodulated RF envelope at the input 75. In the apparatus inwhich the envelope detector circuit 76 is employed, the RF envelope willbe amplitude modulated as a result of the application of the dithersignal to the movable element 32. "Average amplitude" and "withoutamplitude modulation" are used herein to define an RF envelope whoseamplitude is not modulated, except by the dither signal, if such signalis applied to the movable element 32.

Note that unlike conventional automatic gain control circuits, thereference level setting feedback loop 299 herein takes the referencelevel for the detector circuit gain control from the drop out interval102 of the input video signal itself.

In other versions of video record/reproduce systems, the RF envelope maynot have the drop out interval 102 between the RF envelopes 100 (FIG.7a). For example, the system may include two transducing heads and mayinstead generate a continuous RF envelope with no drop out intervalsbetween scans across the tape. In such instances, a drop out interval,wherein the RF envelope is 100% modulated, i.e., has an amplitude ofzero, may be "artificially" generated. By way of example, in FIG. 10a, adiode matrix modulator 323 may be inserted in the continuous RF envelopeinput on line 75 leading to the envelope detector circuit 76, asdepicted in phantom line. The modulator 323 generates a drop outinterval in response to the drum tachometer signal on line 321, wherebyan artificial drop out period is generated identical to the drop outperiod 102 of previous description.

The output of the envelope detector circuit 76 is, in turn, coupled toan active high pass filter 300 which passes signals above about 175 Hzto the synchronous detector 78, when the active filter is connected inthe signal path. A pair of switches 302 and 304 operate to alternativelypass the signal through the filter or bypass the filter as is desired.During initial acquisition of tracking, there may be a 60 Hz componentpresent in the signal that is of much higher amplitude than the dithercomponent of about 450 Hz and the closing of the switch 304 for aboutone second filters the lower frequency component from the signal untilthe desired tracking is achieved, at which time switch 304 opens andswitch 302 closes to bypass the filter 300. The switches 302 and 304 arecontrolled to be in opposite states by the level of the tracking delaysignal placed on line 325 when an operator activates the automatic headtracking control circuitry and the coupling of the signal through aninverter 327 before applying it to the control input of switch 304.

The signal detected by the envelope detector 76 is applied to thesynchronous detector 78 from either switch 302 or 304, and thesynchronous detector has at its other input the phase compensated dithersignal received over line 87 from the commutating comb filter 306 of theautomatic dither signal reference phase compensating means described indetail hereinbelow. The filter 306 separates and phase compensates thedither frequency components of the signal generated by the sense strip83 of the bimorph element 32 and coupled to the filter via line 308 thatis connected to a sensing circuit associated with the element 32 andcontained within the aforementioned electronic dampening circuit 71. Thesensing circuit and its operation is comprehensively described in theaforementioned U.S. Pat. No. 4,106,065 to Ravizza, Ser. No. 677,828.

Referring now to FIG. 12, the head tracking position error signal isdetected by the envelope detector 76 and provided to the synchronousdetector 78. The synchronous detector 78 also receives a phasecompensated reference signal over line 87, which is coupled to itscontrol input. In FIG. 12, like reference numerals identify likecomponents described hereinabove with reference to other figures of thedrawings. The phase compensated reference signal is provided by acommutating comb filter 306 which functions to separate the fundamentaldither frequency component from all other components established in themovable element 32 by inducing a small oscillatory motion in the elementthrough the application of an oscillatory drive signal to the movableelement 32. The oscillatory or dither drive is provided to the movableelement 32 by the dither oscillator 60. As a result of the oscillatorydrive, a vibration is established in the movable element. Only thefundamental frequency component of the vibration is of interest.Therefore, a comb filter 306 is employed to pass the fundamentalcomponent while rejecting all other frequencies generated by movement ofthe element. The frequency filtered by the comb filter 306 is processedinto a reference signal of the proper phase, irrespective of any changesin the mass or other characteristics of the assembly formed of theelement 32 and transducing head 30 that effect the responsecharacteristics of the assembly. This processed reference signal isemployed by the synchronous detector 78 for detecting the head positionerror signal applied to the head position servo circuitry 90.

The sense strip 83 of the movable element 32 is coupled to an input ofthe electronic damping circuit 71 as explained more fully in theaforementioned Ravizza U.S. Pat. No. 4,106,065, which is incorporatedherein by reference. The output signal of the sensing strip 83 isbuffered in the damping circuit 71 and, subsequently, applied to aninput of the filter 306 by means of the line 308. The second output ofthe damping circuit 71 is coupled to one input of the summing circuit 69as described hereinbefore to provide a damping signal of proper phaseand amplitude to the movable element drive amplifier 70 for compensatingextraneous disturbing vibrations induced in the movable element.

The dither signal produced by the oscillator 60 (typically 450 Hz for 60Hz line standard apparatus and 425 Hz for 50 Hz line standard apparatus)is applied to a second input terminal of the filter 306 by means of theline 62, and the system clock reference signal, REF 2H, is applied to athird input terminal of the filter 306 on a line 404. The outputterminal of the filter 306 is coupled to the synchronous detector 78.The remaining circuitry of the apparatus illustrated in FIG. 12functions in the same manner as described hereinabove with reference toFIG. 1.

The commutating comb filter 306 is illustrated in more detail in theblock diagram of FIG. 13. The line 62, which transmits the dither signalto the filter 306, is coupled to the CLEAR input terminal of a counter406; and, the line 404, which transmits the REF 2H clock signal to thefilter, is coupled to the CLOCK input terminal of the counter 406. Thecounter 406 is a binary counter having four output terminal lines 408coupled to four input terminals of a one-of-ten decoder 410. The counter406 and the decoder 410 are illustrated in FIG. 10a (within thedashed-line block 306) with their standard industry designation 74393and 7445, respectively, along with their connecting pin numbersidentified therein.

The output terminals of the decoder 410 are "open" collector terminalsof transistors having the emitter terminals thereof coupled to groundpotential. Also, when an output transistor in the decoder is notselected, a high impedance appears at the corresponding output terminal.

The decoder 410 output terminals (of which there are ten in thisembodiment) are coupled, respectively, to one side of the capacitors C1through C10. The second side of capacitors C1 through C10 are coupled tothe input terminal of a buffer amplifier 412 and to one side of aresistor R10. The second side of the resistor R10 is coupled to the line308. The output terminals of the decoder 410 are each groundedsequentially in response to incremental counts of the counter 406. Thus,each of the capacitors C1 through C10 samples the amplitude of thesensor signal received on the line 308, and the sampled amplitudes areapplied to the amplifier 412. The output of the amplifier 412, which isillustrated by the waveform shown in 14C, is applied to the input of alow pass filter 414.

Frequency components other than that of the dither frequency areincapable of building up the same charge on the capacitors (C1 throughC10) from cycle to cycle. Thus, any charge accumulated on the capacitorsas a result of frequency components other than the dither frequency willbe cancelled out over time. In this manner, the commutating comb filter306 is designed to have a narrow passband of less than one hertzcentered about the dither frequency and any frequency component outsidethat passband will be suppressed. Accordingly, the signal at the outputof the amplifier 412 will have a frequency component equal to the ditherfrequency only. A general discussion of the operation of filters, suchas the combination of counter 406, decoder 410 and capacitors C1 throughC10, may be had by reference to an article entitled "GET NOTCH q'S INTHE HUNDREDS" by Mike Kaufman, which was published in Electronic Design16, Aug. 2, 1974, at page 94.

The low pass filter 414 smooths out the incremental steps in the outputsignal from the amplifier 412, and the output of this filter is appliedto the input of another amplifier 416. The filter 414 causes an unwantedphase delay in the signal. Accordingly, the output of the amplifier 416is applied to a lead network 418 to compensate for this phase delay ofthe signal.

The output of the lead network 418 is applied to a level detectoramplifier 420, and the output of this amplifier is applied to the inputof a limiter 422 having an output terminal coupled to the synchronousdetector 78. The level detector amplifier 420 and the limiter 422operate to shape the phase-corrected and frequency-filtered signalsensed by the sensing strip 83 into a square-wave signal having afrequency and phase corresponding to the mechanical vibrations inducedin the movable element 32 in response to the applied dither signal.Therefore, the synchronous detector 78 is operated in response to theactual mechanical vibrations induced in the movable element in responseto the applied oscillatory dither signal. Accordingly, it may beappreciated that any slight changes in the phase of the mechanicalvibration of the movable element (as may occur when the element isreplaced with another, having a different resonant frequency) willeffectively be automatically cancelled out, thereby eliminating any needfor an operator controlled phase adjustment of the reference signal forthe synchronous detector 78 following a subsequent replacement of themovable element 32, or a transducer head 30 on the element.

To more fully understand the operation of the aforedescribed circuitry,reference is made to the waveforms illustrated in FIGS. 14a through 14f.When the system is operating in a slow motion or still frame mode, theoscillatory motion of the movable element 32 corresponds to the waveformshown in FIG. 14a. Portion 424, which is at the 60 Hz standardtelevision vertical frequency for a single field still motion mode,represents the resetting of the movable element 32 following the scan ofone track to the beginning rescan of the same track. Portion 426 of thewaveform of FIG. 14a represents the oscillatory motion of the movableelement 32 in response to application of the oscillatory dither signal.The portion 426 only of the waveform 424 is filtered by the comb filter306 from the other oscillatory motions, such as that represented by thecomposite waveform 424. It is noted that the dither frequency ispreferrably chosen to be between any of the harmonics of the 60 Hzstandard television vertical frequency so as to avoid spectrum overlap,which overlap would prevent effective filtering of the dither frequencyfrom the vertical frequency. In one embodiment for 60 Hz line standardapparatus, the dither frequency was chosen at 450 Hz, which is betweenthe seventh (420 Hz) and eighth (480 Hz) harmonics of the verticalfrequency. However, the dither frequency need not be at the precisemidpoint between vertical frequency harmonics; but may be chosensubstantially between such harmonics so long as there is no possibilityof spectrum overlap. This may be more fully appreciated by the frequencyspectrum diagram of FIG. 15.

When the apparatus is operating in the normal speed mode, theoscillatory motions of the movable element 32 correspond to the waveformillustrated in FIG. 14b. Locations 428 in the illustrated waveformidentify the periodicity of the same vertical frequency to be suppressedby the comb filter 306. Here, as in the waveform of FIG. 14a, it is thedither frequency components of the element oscillatory motions that areto be filtered from all other oscillatory motion frequency components ofthe movable element 32.

It is noted that the waveforms shown in FIGS. 14c through 14f areillustrated on an expanded time scale for clarification purposes only,and should not be confused with the periodic relationships of thewaveforms shown in FIGS. 14a and 14b. The waveform shown in FIG. 14crepresents the signal appearing at the output of the buffer amplifier412, while that shown in FIG. 14d represents the signal appearing at theoutput of the low pass filter 414. Note that the waveform in FIG. 14d isdelayed in phase from that shown in FIG. 14c. This phase delay, asstated above is caused by the low pass filter 414.

The waveform shown in FIG. 14e represents the output signal from theamplifier 420, and is shown to be back in phase with the signalrepresented by FIG. 14c. The waveform shown in FIG. 14f represents theoutput signal from the limiter 422, which is the wave-shaped andphase-corrected reference signal applied to the synchronous detector 78.

The output of the synchronous detector 78 provides the DC error signalwhich is applied to an error amplifier servo compensation network 310shown in FIGS. 10a and 10b and the DC error signal appears on line 80that is applied to switches 120 and 122 as previously mentioned. Thecircuit 310 includes a disable switch 312 that is controlled by line314, which line is also coupled to control another switch 316 in thecorrection signal output buffer circuitry 329, which includes themovable element's drive amplifier 70. The line 314 is also coupled to aswitch 318 associated with the level detectors 156, 157, 158 and 160.The switches 314, 316 and 318 are operative to disable the circuits withwhich they are associated and such is done when it is not desired thatthe automatic head tracking circuitry be operating. For example, whenthe tape is being shuttled at a very fast rate, a low logic level WINDDISABLE signal is placed on line 432 as a result of an operatorinitiated shuttle command being provided to the record/reproduceapparatus. During such operations, it is essentially impossible for theautomatic head tracking circuitry to lock onto a track. Therefore, it isdesired that the automatic head tracking circuitry be disabled and line314 is controlled through the logic circuitry shown in FIGS. 10a and 10bwhen the operating condition of the video record/reproduce apparatus isplaced in a high speed shuttle, as determined by the operator. When theoperator terminates the shuttle, the WIND DISABLE signal goes to a highlogic signal level and the disable signal is removed from the switches.The input signals on lines 283, 285 and 287 to the circuitry shown inFIGS. 10a and 10b also, dictate that the switches to be set to disablethe automatic tracking circuitry. The line 283 receives a logic levelstate signal indicative of whether the operator has initiated operationof the automatic head tracking circuit. The lines 285 and 287 receivelogic level state signals according to whether the record/reproduceapparatus is in a capstan tach phase lock operating mode or a slow/stillor acceleration operating mode, respectively. These logic level statesignals are received from the portion of the capstan servo circuitryshown in FIGS. 11a, 11b and 11c.

The circuitry for providing reset pulses to the AND gates 140, 142 and144, as well as the color frame verification circuitry 340 described infurther detail hereinbelow, includes line 182 which extends to the clockinput of the latches 170, 172 and 174, to the color frame verificationcircuitry 340 and to the pulse and clock generator circuitry 184. Thegenerator circuitry 184 produces the reset pulses on line 186 thatextend to and are passed by any of the gates 140, 142 and 144 that areenabled by their associated latch. The pulse and clock generatorcircuitry 184 includes a two stage flip-flop circuit 324 that has itsclock input coupled to the not true output of a one-shot 331 that servesto delay the generation of the reset pulses so that they coincide withthe occurrence of the drop out interval 102 (FIG. 7a). Morespecifically, the one-shot 331 receives the processed drum tach signalcoupled to its clock input by line 182 at a time before the occurrenceof the drop out interval 102 of about 0.67 msec., which, as describedhereinbefore, is at the reset decision time identified in FIG. 7 by thereference number 108. The timing circuit of the one-shot 331 is set bythe adjustment of the reset potentiometer 333 to have a period thatproduces a 0.67 msec. negative pulse at its not true output. Thepositive going trailing edge of the negative pulse is coupled to theclock input of the first stage of the flip-flop circuit 324, whichresponsively conditions the second stage so that, upon the occurrence ofthe next reference 2 H pulse received over line 322 from the studioreference source, the flip-flop circuit removes an inhibiting signalplaced on the clear input, CLR, of a counter 326. In addition, theflip-flop circuit 324 switches the opposite phased signal levels placedon lines 186. Following the removal of the inhibiting signal from itsclear input, CLR, the counter 326 counts the 2 H pulses received overline 322 until it reaches its terminal count, which takes a time of 512microseconds. At this time, the count provides a signal to the flip-flopcircuit 324 that clears it, which returns the flip-flop circuit to itsstate that provided an inhibiting signal to the counter by switchingsignal levels on lines 186 back to the levels that existed prior to thereceipt of the processed drum tach signal. This switching of the signallevels on lines 186 serves to generate the reset pulses that are coupledto the AND gates 140, 142 and 144 each time a processed drum tachoccurs. A reset pulse is passed by an AND gate to the integrator 134 forresetting the voltage level on its output line 66 whenever the AND gate(or AND gates if a two track forward reset is called for) is enabled byits associated latch.

The three threshold reference levels for the level detector 158 that areproduced by the variable reference circuit 126 are shown in FIG. 10a asbeing produced by the operation of open collector gates 328 and 330,which are in turn controlled by the control lines 118a and 118b fromlogic gates 332. The logic gates control the open collector gates 328and 330 in accordance with the conditions of the slow/still, 95% normalspeed and normal speed operating mode related input signals applied tothe logic gates, which appear on mode control lines 285 and 287 and atthe output of the inverter 450, as shown in FIGS. 10a and 10b. Each ofthe gates 328 and 330 is of the type which effectively apply a low logicsignal level at its output when it receives an enabling high logicsignal level at its input and, depending on which, or if both of thegates are enabled, results in a different voltage being applied on line196 which extends to the level detector 158. More particularly, whengate 330 receives a high logic signal level at its input (caused by aSLOW/STILL low logic signal level on mode control line 287 during thevelocity ramp and slow/still operating modes), then line 196 isessentially grounded (low logic signal level) to set the thresholdreference level for the level detector 158 at a point corresponding tono head deflection in the reverse direction. If gate 328 receives a highlogic signal level at its input (caused by an AST tach low logic signallevel on mode control line 285 during the 95% normal speed mode and theabsence of 100% tach pulse at the input of the inverter 450 during the100% normal speed mode, i.e., during the entire capstan tach phase lockmode), then its output is essentially grounded and resistors 334 and 336comprise a voltage divider network which applies an intermediate voltageon line 196. This sets the threshold reference level for the leveldetector 158 for the 95% normal speed operating mode i.e., at a pointcorresponding to a head deflection in the reverse direction of justgreater than (about 10% more than) one-half the separation of adjacenttrack centers. If neither of gates 328 and 330 receives a high logicsignal level at their respective inputs (when in operating modes otherthan slow/still and 95% normal), then a high voltage (high logic signallevel) appears on line 196. The high voltage on line 196 disables thevariable reference level detector 158. With the level detector 158disabled, only the fixed threshold reference levles associated with thelevel detectors 156 and 160 control the repositioning of the movablehead in the normal speed mode. From the foregoing, it can be seen thatthe open collector gates function together with the source of fixedthreshold reference levels to selectively cause the generation headpositioning reset pulses in accordance with the operating mode of theapparatus.

The output of the integrator 134 appears on line 66 which extends to thelevel detectors 156, 157, 158 and 160 for monitoring and, through gainadjusting switch 337, through an AC and DC correction adder circuit 338and finally to the output buffer circuit 329 for application to thesecond summing circuit 69 and eventually the movable element 32 (FIG.12). The added AC error correction signal is derived from the output ofthe error amplifier network 310 present on line 80a. The errorcorrection signal provided by the error amplifier network 310 containsAC and low rate, or DC components. Line 80a extends to a band selectivefilter (not shown) such that the comb filter employed in the apparatusdescribed in the above-identified Ravizza, et al application Ser. No.669,047, to obtain the AC error component from the composite errorsignal. The AC error signal provided by the comb filter is coupled tothe adder circuit 338 via input line 80b. The AC and DC head positionerror signals are summed together by the adder circuit 338 and thesummed head position error signal is coupled by line 66a to the firstsumming circuit 64 for combining with the dither signal provided by thedither oscillator 60. The output of the first summing circuit 64 iscoupled by the buffer circuit 329 to line 68 that extends to the secondsumming circuit 69, which adds the dampening signal provided by theelectronic dampening circuit 71 (FIG. 12) to form a composite headposition error correction signal for driving the movable element 32 viathe drive amplifier 70.

A color frame verification circuit 340 shown in FIG. 10a verifieswhether a correct initial color frame determination was made and, in theevent the movable head 30 is scanning the wrong track for proper colorframing, effectively causes it to be deflected to the proper trackbefore initiating normal reproduction operations in the normal speedmode. The color frame verification circuit 340 is enabled during the100% normal speed operating mode just prior to synchronous reproductionoperations by the 100% TACH signal provided by the logic circuitry 224shown in FIGS. 11b and 11c. This occurs at the time that the control ofthe transport servo is switched from the capstan tach servo phase lockmode to the control track servo phase lock mode.

A signal entitled "Field Mismatch", which is coupled to one of twoinverting input terminals of an AND gate 441, is derived by the fieldmatch generator 95 (FIG. 2) of the apparatus from the video transducinghead output and not from the control track read head. The field mismatchsignal is derived from a comparison between the video tracks beingreproduced by the apparatus and reference signals provided by a user ofthe apparatus, such as conventional studio reference signals. Circuitryfor deriving the field mismatch signal is typically found in helicalscan video record/reproduce apparatus, such as the aformentioned VPR-1video production recorder. As previously explained, if a wrong initialcolor frame determination has been made, the movable element 32 will bein an erroneous deflected position for proper color frame conditions.The color frame verification circuit takes advantage of the conditionthat, if a wrong initial color frame determination has been made, theincorrect monochrome field will be reproduced. Briefly, however, amonochrome field mismatch is determined by applying the studio referencevertical signal to the data (D) input of a first flip-flop, and thestudio reference horizontal signal to the clock (C) input terminal ofthe same flip-flop. Likewise, the vertical and horizontal signalsreproduced by the transducing head 30 of the apparatus are applied tothe data (D) and clock (C) input terminals of another flip-flop. Thetrue (Q) output terminals of these two flip-flops are coupled to twoinput terminals of an EXCLUSIVE OR gate, and the output of this gatecomprises the field mismatch signal referred to herein. The output ofthe EXCLUSIVE OR gate is in opposite states for monochrome field matchand mismatch conditions. In the apparatus herein, a low logic level atthe input of the AND gate 441 signifies that an erroneous monochromefield match exists, hence, the initial color frame determination waserroneous and a high logic level that a monochrome field match exists,hence, a correct determination was made.

When a field mismatch occurs, circuitry 340 applies a reset step to themovable element input buffer circuit 329 to move the transducing head tothe proper track. Alternatively, the capstan drive could be pulsed tomove the tape 36 so as to position the head 30 adjacent the proper trackas is the practice in the prior art. However, it is virtually impossiblein commercially practicable tape record/reproduce apparatus toaccelerate and decelerate the tape 36 in the short time alotted (about0.5 msec.) to reposition the tape within the drop out period and,therefore, it is common to experience disturbances in the display ofprior art record/reproduce apparatus when the tape is slewed to correcta field mismatch.

The output terminal of the gate 441 is coupled to the data (D) inputterminal of a flip-flop 442 and to the inverting clear (CLR) inputterminal of this same flip-flop. The true (Q) output terminal of theflip-flop 442 is coupled to the data (D) input terminal of a flip-flop444. The true (Q) output terminal of the flip-flop 444 is coupled backto the second inverting input terminal of the AND gate 441, therebyforming a latch that comprises gate 441 and the flip-flops 442 and 444.

A signal entitled "Video Record", which is at a low level when theapparatus is in a record mode of operation and at a high level during areproduce mode of operation, is applied to an input terminal of aone-shot 446. The true (Q) output terminal of the flip-flop 446 iscoupled to one of two inverting input terminals of a NOR gate 448.Another input signal entitled "100% TACH" 502 (FIG. 16) provided by thetape transport servo of the apparatus when switched to the 100% normalspeed in the capstan tach lock mode, is coupled to an input terminal ofan inverter 450. The output of the inverter 450 is coupled to one of twoinverting input terminals of the AND gate 332, and to the secondinverting input terminal of the NOR gate 448.

The output terminal of the NOR gate 448 is coupled to the positivetrigger input terminal of a one-shot 452. The output terminal of theone-shot 452 is coupled to the clock (C) input terminal of the flip-flop442 and to the inverting clear (CLR) input terminal of the flip-flop444. Accordingly, a trailing positive edge transition 503a (FIG. 16) atthe conclusion of the 100% TACH signal 502 will trigger the one-shot 452by means of the inverter 450 and the NOR gate 448.

Assume for the present discussion that the flip-flops 442 and 444 arereset, and that a field mismatch has been detected by the field matchgenerator 95. The output of the AND gate 441 will be at a high level,and the triggering of the one-shot 452 will clock the flip-flop 442 intoa set state to enable the AND gate 456 to respond to the receipt of aninverted processed drum tach at the output of the inverter 454.

The processed drum tach signals 510 (FIG. 16), which are supplied on theline 182, are applied to the input terminal of an inverter 454 and theoutput of this inverter is coupled to the clock (C) input terminal ofthe flip-flop 444 and into one of two inverting input terminals of anAND gate 456. The not true output terminal of the flip-flop 442 iscoupled to the second inverting input terminal of AND gate 456. Theoutput terminal of the AND gate 456 is coupled to one of two inputterminals of each of NAND gates 458 and 460. When the flip-flop 442 isin a set state, as described above, the processed drum tach signal isinverted by the inverter 454 and gated through the AND gate 456 to inputterminals of the NAND gates 458 and 460. On the positive-going trailingedge of this tach signal, the flip-flop 444 is set which disables theAND gate 456. Consequently only one setting pulse is applied to the NANDgates 458 and 460 in response to the single negative transition of thefield mismatch signal.

The output of the level detector 157 (FIG. 10b), indicating the positionof the movable transducing head, (that is whether or not the head isdeflected in either the forward or reverse direction a distancecorresponding to the separation of adjacent track centers after theinitial color frame determination is complete), is provided on line 159;and this line is coupled to the second input terminal of the NAND gate458 (FIG. 10a) and to the input terminal of an inverter 462. The outputterminal of the inverter 462 is coupled to the second input terminal ofthe NAND gate 460. The output terminal of the NAND gate 458 is coupledto the inverting set (S) input terminal of the latch 170. Similarly, theoutput terminal of the NAND gate 460 is coupled to the inverting set (S)input terminal of the latch 174. The single setting pulse, generatedfrom the processed drum tach signal and provided by NAND gate 458 or 460for displacing the head one track, if one of these NAND gates is enabledby the signal level appearing on the line 159, causes the generation ofa single appropriate reset pulse for displacing the head 30 one track inthe appropriate direction for proper color framing as will be furtherdescribed below.

Following the generation of a reset pulse for effecting therepositioning of the movable head 30, a field reference pulse,designated FIELD REF, generated by a conventional tachometer processingcircuitry, is provided on line 464 and is coupled to the clear inputterminal of the latches 170, 172 and 174. The field reference pulse isderived from the once around drum tachometer pulse and is timed to occurabout 1/120 of a second following the tachometer pulse. Upon theoccurrence of the field reference pulse, each of the latches is placedin its clear state, thereby, removing the enabling input from theassociated AND gates 140, 142 and 144. Furthermore, in the modified formof the automatic head tracking servo circuitry described in detailhereinafter with reference to FIGS. 10c and 10d, the field referencepulse is coupled to also clear the additional latches provided for NTSC,PAL and SECAM color frame still motion modes of operation.

To more fully understand the operation of the aforedescribed circuitry340, reference is made to FIG. 16, wherein a timing diagram illustratingoperation of the track selection logic is illustrated. Waveform 500illustrates the same tape velocity versus time profile shown in FIG. 9and described hereinabove. Waveform 502 illustrates the 100% TACH signalapplied to the input terminal of the inverter 450. Portion 503 of thewaveform 502 is approximately a 0.6 second window produced by a one-shot371 included in the logic circuitry 224 illustrated in FIG. 11b, whichis triggered in response to the capstan 200 reaching 100% normal speed.

Waveform 504 is a diagram of the changing track reset conditions duringthe transitory period of speed changing as illustrated by the waveform500. The time periods 504a, 504b and 504c correspond to the threedifferent modes of operation illustrated in FIGS. 7d, 7e and 7f,respectively, and described hereinabove. During the time periodcorresponding to the portion 503 of the waveform 502, a track resetwindow is opened to plus or minus one track reset range so that if themovable head 30 is mispositioned after the initial color framedetermination in the reverse (or forward) direction by one trackposition, it will not be reset forward due to the threshold levelprovided to the level detector 158 as the automatic head tracking servocircuitry operates to correct the mispositioned head 30.

Waveform 506 illustrates the signal at the true (Q) output terminal ofthe one-shot 452 during this transitory time period. The leading edge507 of the pulse portion of the waveform 506 is timed to trailing edge503a of the pulse portion 503 of the waveform 502.

Waveform 506' is the waveform 506 shown in expanded time scale forclarification purposes only. Waveform 510 illustrates the processed drumtach signal applied at the input terminal of the inverter 454 andwaveform 512 illustrates an erroneous monochrome field mismatch, hence,erroneous initial color frame determination, and the following highlevel of the same signal illustrates a corrected monochrome fieldmismatch. Edge 513 is the result of correcting the monochrome fieldmismatch error that was represented by the low-level signal state at theinput of the AND gate 441. The edge 513 coincides with the vertical syncof the reproduced signal (not shown), which is approximately 0.5 msecafter the occurrence of edge 511b of the processed drum tach pulse 511that initiates the one track head positioning step for correcting thefield mismatch.

Waveform 514 illustrates the signal appearing at the true (Q) outputterminal of the flip-flop 442 as a result of the presence of a fieldmismatch when the apparatus is switched to the normal speed mode. Whenthe waveform 512 is at a low level and waveform 506 makes a transitionto a high level (i.e., at leading edge 507), the flip-flop 442 sets atleading edge 515. Waveform 516 illustrates the signal apearing at theoutput of the AND gate 456 in response to the above-described signals.In response to leading edge 515 of the pulse signal 514, the AND gate456 is enabled to pass a setting pulse 517 to enable the setting oflatch 170 or 174 as determined by the state of the forward/reversesignal supplied on the line 159 by the level detector 157 as a result ofthe voltage level on line 66 at the output of the integrator 134. Thatis, if the transducing head 30 is mispositioned at the conclusion of theinitial color frame determination in the reverse direction by one trackposition, the level detector 157 of the color frame verificationcircuitry 340 detects an erroneous initial color frame determination andeffects a one track forward field mismatch correcting reset movement ofthe movable element 32. Conversely, if the transducing head 30 ismispositioned in the forward direction by one track position, it isdetected by the level detector 157 and circuitry 340 effects a one trackreverse field mismatch correcting reset movement of the element.Accordingly, if the transducing head 30 is detected as being on thewrong track after the initial color frame determination, that is a fieldmismatch condition, the appropriate one of the NAND gates 458 or 460 isenabled by the signal level placed on the line 159 by the level detector157, and the enabled NAND gate passes the setting pulse 517 to the set(S) terminal of the appropriate one of the latches 170 or 174, if asetting pulse 517 is provided by the AND gate 456. By setting one of thelatches 170 or 174, the associated AND gate 140 or 144 is enabled and asdescribed hereinbefore, this places a reset pulse on line 186 to becoupled to the integrator 134 for resetting the head 30 the necessaryone track forward or reverse direction as required to obtain propercolor frame field match. The direction of the reset is determined by theposition of the head 30 at the occurrence of the leading edge 517a ofthe setting pulse 517.

Should the initial color frame determination be correct, the resultinghigh level of the field mismatch signal 512 at the input of the AND gate441 disables the color frame verification circuit 340 and the AND gate456 does not provide a setting pulse 517 to the latches. Hence, the head30 is allowed to remain in the same position after the initial colorframe determination as it was at the determination.

During the time frame encompassed by pulse portion of the waveform 506(time duration of the one-shot 452) numerous processed drum tach pulses(waveform 510) occur. As briefly discussed above, only a single resetstep should be applied to the movable element 32 to correct for a singledetected one track mispositioning of the head 30. To this end, theflip-flop 444 operates to lock out the additional processed drum tachpulses during the color frame correction period as described above.Waveform 518 illustrates the true (Q) output signal of the flip-flop 444which is applied to the input of the AND gate 441. The pulse 517coincides with the processed drum tach pulse 511. The processed drumtach pulse 511 is expanded in time for sake of clarification of thedescription. The leading edge 520 of the waveform 518 provided at theoutput of the flip-flop 444 coincides with the trailing edge 511b of thetach pulse 511. This resets the latch comprising the AND gate 441 andflip-flops 442 and 444, which disables the AND gate 456, therebyinhibiting any additional setting pulses (waveform 516) being applied tothe NAND gate 458 or 460. The trailing edge 521 of the waveform 518coincides with the trailing edge 508 of the waveform 506 as a result ofthe one-shot 452 being timed out. This defines a color frame correctionhead track adjustment window of about 0.25 second, after which nofurther reset pulses are applied to the integrator 134 by the colorframe verification circuit 340. This condition remains until anothercolor frame correction is required.

Changes in the head to track positioning error exceeding the bandwidthof the automatic head tracking servo circuitry will not, of course, beprocessed and, hence, not corrected. Operating characteristics of theparticular video record/reproduce apparatus, for which the automatichead tracking servo illustrated by FIGS. 10a and 10b is designed,dictated that a servo bandwidth of 30 Hz was preferred. However, someoperating conditions of the video record/reproduce apparatus can resultin the head 30 being mispositioned so that the resulting trackpositioning error signal is at a rate that exceeds the 30 Hz servobandwidth. For example, when the video record/reproduce apparatus is inthe still frame operating mode, the automatic head tracking servo mayinitially provide a head positioning signal on line 66 (FIG. 3) thatcauses the head 30 to be mispositioned so that at the start of the scanof the tape 36 the head starts its scan over one track, crosses theguard band between adjacent tracks and ends its scan over an adjacenttrack. Under these circumstances, the track crossing of the head 30produces a 60 Hz error signal and the head tracking servo will be unableto respond to correct the head's misposition. Instead the head trackingservo would act as if the head 30 is correctly positioned and, thereby,issue an output signal that leaves the head 30 mispositioned. As aresult of such cross-tracking, the resulting RF envelope reproduced bythe transducing head 30 shrinks in amplitude to a minimum amplitude whenthe head crosses the center of the guard band. Because of limitedbandwidth of the servo circuit, a transient reset pulse is produced bythe integrator 134 in the head positioning signal on line 66. Thistransient reset pulse typically is of insufficient amplitude to triggerthe reset of the movable element 32. Accordingly, the servo system is inan ambiguous state of scanning portions of two adjacent tracks as aresult of not resetting the position of the movable element 32 for arescan of the first of the two adjacent tracks. The scanning path 105followed by the head 30 along the tape 36 under such circumstances isdepicted in phantom line in FIG. 6.

A disturbance in the head positioning servo circuitry or in thedeflection of the movable element can also lead to permanent headmispositioning. If the disturbance is synchronous with the timing ofalternate resets of the head position during a still frame mode so thatsuch resets are not performed, the head positioning servo circuit willallow the head to scan two adjacent tracks in succession and then issuea two track forward reset step to the movable element 32. The two trackforward reset step is issued because after the scan of the second of thetwo consecutively scanned tracks, the head positioning signal providedon line 66 by the integrator 134 is in excess of both the 0 and 2 trackforward reset threshold levels of the level detectors 158 and 156 (FIG.3). Consequently, as previously described, a two times amplitude resetpulse is provided to the integrator 134. As long as the synchronousdisturbance persists, the movable element 32 will be controlled by theautomatic head positioning servo to repetitively scan two adjacenttracks. If the image information contained in the two video fieldsreproduced from the two tracks contains relative movement, a horizontaljitter will appear in the displayed signal. The head positioning signalprovided by the integrator 134 under such condition is depicted in theconnected phantom lines 103 and 104 in FIG. 7c.

Ambiguous track lock resolving circuitry 342 (portions in both FIGS. 10aand 10b) prohibits the servo system of the apparatus from locking in theaforementioned ambiguous states when the video record/reproduceapparatus is operating in a still frame mode. The circuitry 342 isdisposed for detecting such a reset failure at the end of a scan of asingle track. One-shot 343, having an input terminal coupled to receivea signal on input line 339 derived from the reproduced control trackpulses 94 detects the absence of tape motion such as occurs during thestill frame mode of operation. The output of one-shot 343 is coupled toone of two input terminals of a NAND gate 345, and the output terminalof this NAND gate is coupled to the set input terminal of the latch 172.

The not true output terminal of the latch 172 is coupled to one of twoinput terminals of the AND gate 142, and the second input terminal ofthis AND gate is coupled to receive, over one of the lines 186, thereset pulse from the not true output terminal of the flip-flop circuit324 located within the pulse and clock generator circuit 184. In thestill frame operating mode, the output terminal of the gate 142 shouldproduce a reset pulse for stepping the movable element 32 every headrevolution. In addition, the output of the AND gate 142 is coupled tothe negative trigger input terminal of a one-shot 347, and the trueoutput terminal of this one-shot is coupled to one of two inputterminals of a NAND gate 349. The positive trigger input terminal of theone-shot 347 is coupled to +5 volts, and the one-shot time duration isdetermined by the time constant of the associated resistor/capacitornetwork coupled to pins 14 and 15 of this one-shot. The not true outputterminal of the one-shot 347 is coupled to a set input terminal ofanother one-shot 351.

The embodiment shown in FIGS. 10a and 10b is arranged for controllingthe tracking position of the scanning head 30 when NTSC standardtelevision signals are recorded and reproduced by the apparatusdescribed herein. Modifications of the automatic head tracking servoshown in FIGS. 10a and 10b for controlling the scanning head's trackingposition when other television signal standards, such as PAL and SECAM,are recorded and reproduced by the apparatus described herein are shownin FIGS. 10c and 10d. For NTSC television signals, one-shot 347 is setfor a timing of approximately 25 msecs., and the one-shot 351 is set fora timing of 160 msecs. Thus, the resulting 25 msec. pulse provided bythe one-shot 347 is greater than the interval between consecutive resetpulses provided to the AND gate 142, and less than the time requiredbetween three consecutive reset pulses. As described hereinbefore, areset pulse is provided by the pulse and clock generator circuit 324 foreach revolution of the head 30, hence, at a frequency of a 60 Hz.Consequently, if a reset pulse is not provided at the output of the ANDgate 142, the one-shot 347 will time out and, thereby, set the one-shot351 and condition the NAND gate 349. The setting of the one-shot 351corresponds to the time required for approximately ten consecutive resetpulses. Conditioning of the NAND gate 349 in response to setting of theone-shot 351 will condition the NAND gate 345, which will hold the latch172 in a set state for the approximately ten reset pulse time period.Accordingly, ten consecutive reset pulses will be provided at the outputterminal of the AND gate 142, at the proper times for such reset pulses,to thereby reset the output of the integrator 134 an amount equivalentto a forward 1 track deflection of the movable head 30 and force theservo system out of the ambiguous state.

The modifications to the automatic head tracking servo circuitry shownin FIGS. 10a and 10b to condition the circuitry for still modeoperations during which multiple fields are reproduced from a pluralityof tracks and to condition the ambiguous track lock resolving circuitry342 for proper operation with a signal standards other than NTSC, asbriefly discussed hereinabove, are illustrated in FIGS. 10c and 10d. Theillustrated modifications permit operations with PAL and SECAMtelevision signals. The line 182, which transmits the processed drumtach signal, is coupled to the clock input terminal of an 8-bit dividercircuit 380 formed of three flip-flops 381, 382 and 383 coupled in aconventional cascaded manner. Also, the line 182 is coupled to aposition 1 contact terminal of a switch 384. The output terminals of theflip-flops 381, 382 and 383 are coupled to position 2, 3 and 4 contactterminals of the switch 384. The operating terminal of the switch 384 iscoupled to junction 183 along line 182, which extends to the resetenabling latches associated with the integrator 134, flip-flop circuit324 and color frame verification circuitry 340 (FIG. 10a). The "FieldMismatch" signal, as discussed above, is applied to the inverting clearinput terminals of the flip-flops 381, 382 and 383 to inhibit operationof the divider 380 until a field match condition exists. Changing theposition of the movable contact of the switch 384 results in changingthe number of processed drum tach pulses required to be received overline 182 before a reset pulse is provided to the AND gate circuitryconnected to line 132 extending to the integrator 134 (FIG. 3). Thispermits the frequency of the reset signal provided to the integrator 134to be selectively varied for different still frame modes.

Switch 384 is mechanically coupled to switches 386 and 387, havingoperating terminals thereof coupled to the +5 volt supply. Positions 1-4of the switches 384, 386 and 387 correspond to one another so that whenswitch 384 is in position 1, switches 386 and 387 are also inposition 1. The position 1 contact terminal of the switch 386 is coupledto pin 15 of the oneshot 347 through resistor R20, and position 1 of theswitch 387 is coupled to pin 7 of the oneshot 351 through resistor R22.The values for the resistors R20 and R22 are the same as that discussedabove to provide a 25 msec. time duration for oneshot 347 and a 160msec. time duration for oneshot 351. With the movable contacts ofswitches 386 and 387 in position 1, the ambiguous track lock resolvingcircuitry is arranged for operation in the still frame mode wherein asingle field is repetitively reproduced to generate a still display.

The three contact terminals (positions 2, 3 and 4) of switch 386 arecoupled through resistors R24, R26 and R28, respectively, to pin 15 ofthe oneshot 347. Positions 2, 3 and 4 of the switch 387 are similarlycoupled through resistors R30, R32 and R34 to pin 7 of the oneshot 351.The values for the resistors R26, R28 and R30 are selected to providetime durations of 46 msec., 82 msec. or 170 msec., respectively, of theoneshot 347. Similarly, the values for the resistors R30, R32 and R34are selected to provide time durations of 320 msec., 640 msec. or 1280msec., respectively, of the oneshot 351.

With the movable contact of the switches 386 and 387 respectively in oneof the positions 2, 3 and 4, the ambiguous track lock resolvingcircuitry 342 is arranged for operation in one of the still frame modes,wherein a two (for monochrome frame), four (for NTSC or SECAM colorframe) or eight (for PAL color frame) field sequence, respectively, isrepetitively reproduced to generate a still display.

The values of the capacitors bridging the pins 15 and 14 of the oneshot347 and pins 7 and 6 of the oneshot 351 remain unchanged in thisembodiment. However, the capacitors could also be switched whilemaintaining the value of the resistors constant, or both the capacitorsand resistors could be conjointly changed, to change the time constantsof the oneshot circuits as required for the desired still frameoperating mode.

When switches 384, 386 and 387 are in positions 2, 3 or 4, the processeddrum tach pulses are divided by two, four or eight, respectively.Accordingly, the position of the transducing head 30 will be reset afterscanning the second, fourth or eighth consecutive field of the recordedinformation as selected by the mechanically coupled switches 384, 386and 387. However, the amplitude of the reset signal applied to themovable element 32 is correspondingly selected by the thresholdcircuitry operated in conjunction with the associated latches and gatesas shown in FIG. 10d and described in greater detail hereinbelow.Because the movable contact of the switch 384 is ganged to operate withthose of switches 386 and 387, the proper divided process drum tachsignal is provided in the selected still frame mode for effectingissuance of the correcting head position reset signal to the movableelement 32.

Thus, it may be appreciated that when the apparatus is operating in thestill frame mode, an operator places the switches 384, 386 and 387 inposition 1 for scanning a single field between resets of the transducinghead 30. If, however, it is desired to scan two consecutive fieldsbetween resets of the head, such as for a complete monochrome frame, theoperator places these switches in position 2. Position 3 of theseswitches will cause the transducing head 30 to scan four consecutivefields between resets which will produce a complete NTSC color frame, ora jitter-free color frame for SECAM television signals. The position 4of these switches will cause the apparatus to produce a complete colorframe from PAL television signals, when such signals are recorded on thetape.

The modified circuitry for generating the appropriate reset pulse ofcurrent that is coupled by line 132 (FIG. 3) to cause the integrator 134to effect a correspondingly appropriate reset of the head position forthe various single and multiple field still mode operations is shown inFIG. 10d. In the same manner as described hreinbefore, the variablethreshold reference source 126 establishes head reset determingthreshold voltage levels for the level detector 158 and associated ANDgate 142 that generates, in response to the head deflection signal levelon line 66, the appropriate forward head position reset current pulseplaced on line 132 for operating modes below normal speed. Also, thelevel detectors 156 and 160 receive the fixed threshold voltage levels 1track reverse and 1 track forward, respectively, for effecting theappropriate reset of the movable head 30 as described hereinbefore. Forstill mode operations, wherein a single television field is repetitivelyreproduced from the tape 30, the level detector 158 receives a thresholdvoltage from the reference source 126 corresponding to any headdeflection in the reverse direction. At the occurrence of each processeddrum tach pulse, the movable element 32 carrying head 30 will be in adeflected condition corresponding to reverse direction head deflectionat the conclusion of the scan of the track by the head. Therefore, thelevel detector 158 enables the latch 172 which, when clocked places anenabling signal on one of the inputs of the associated AND gate 142,which passes the following reset pulse coupled to its other input byline 186 that extends from the flip-flop circuit 324 (FIG. 10a) of thepulse and clock generator 184 (FIG. 3). The single reset pulse pased bythe AND gate 142 is converted by the resistor 148 to a pulse of currenton line 132 at the conclusion of each revolution, hence, scan of atrack, by the head 30, or at a frequency of 60 Hz in a 60 Hz field ratestandard and at a frequency of 50 Hz in a 50 Hz field rate standard.This effects a 1 track forward reset of the head so that it rescans thetrack during its next revolution. As long as the record/reproduceapparatus is in the single field, still motion mode, the head 30 isrepetitively reset by reset pulses of current generated by the AND gate142 and associated resistor 148, whereby a single television field isrepetitively reproduced from a repetitively scanned track.

For monochrome frame (composed of two interlaced odd and even televisionfields), still motion operating modes, level detectors 156 and 158,together with associated latches 170 and 172, AND gates 140 and 142 andcurrent forming resistors 146 and 148, function to provide a two trackforward reset current pulse over line 132 to the integrator 134, whichresponsively causes the repositioning of the movable head 30 after everytwo revolutions of the head to the track containing the first field ofthe repetitively reproduced two field sequence. This is accomplished byplacing the movable contact of the switch 384 at the output of the eightbit divider circuit 380 (FIG. 10c) in position 2. With the switch 384 sopositioned, the eight bit divider circuit 380 provides frequency dividedprocessed drum tach pulse and reset pulse on lines 182 and 186,respectively, at the completion of every second revolution of the trackscanning head 30, or at a frequency of 30 Hz in a 60 Hz field ratestandard and at a frequency of 25 Hz in a 60 Hz field rate standard.

Since the reset current pulses will be provided to the integrator 134after every two revolutions of the head 30, the integrator will providea head deflection ramp signal, lasting for two head revolutions betweenconsecutive reset current pulses, that deflects the movable element 32 adistance in the reverse direction corresponding to the distanceseparating three adjacent track centers. Therefore, upon the occurrenceof the frequency divided processed drum tach pulse on line 182, bothlevel detectors 156 and 158 are conditioned by the signal level on line66 exceeding the threshold levels established for the latches, asdescribed hereinbefore, to provide signals on lines 164 and 166,respectively, coupled to the D input of the latches 170 and 172 thatenable the following associated AND gates 140 and 142 to pass frequencydivided reset pulses when received over line 186. As describedhereinbefore with reference to FIG. 3, the two reset pulses passed bythe AND gates 140 and 142 are converted to corresponding current pulsesby the resistors 146 and 148 and added together to produce a two trackforward reset current signal on line 132. The two track forward resetsignal causes the head deflection signal on line 66 to be reset and,thereby effect a two track forward deflection of the movable element 32after each reproduction of a two field sequence. In this manner, amonochrome frame still image is provided by the record/reproduceapparatus for all television signal standards.

For color frame still motion operating modes with NTSC and SECAMstandard signals, four consecutive television fields are repetitivelyreproduced in sequence to form the still motion color image. In thesemodes, a level detector 550, together with associated latch 552, ANDgate 554 and resistor 556 connected to the output of the AND gate 554,function together to provide an additional two track forward resetcurrent pulse over line 132 to the integrator 134. The impedance valueof resistor 556 is selected to be one-half the value of the resistors146 and 148 (resistors 146 and 148 being of equal value) so that asingle reset pulse passed by AND gate 554 will be converted to a twotrack forward reset current pulse on line 132. In these still framemodes, AND gates 140 and 142 together also cause a two track forwardreset current pulse to be provided over line 132, which is added to theadditional two track forward reset current pulse to form a four trackforward reset current signal for effecting a repositioning of the head30 after four revolutions. The integrator 134 responds to the four trackforward reset current signal on line 132 to cause the repositioning ofthe movable head 30 to the track containing the first field of arepetitively reproduced four field sequence after every four revolutionsof the head. This is accomplished by placing the movable contact of theswitch 384 at the output of the eight bit divider circuit 380 (FIG. 10c)in position 3. With the switch 384 to positioned, the eight bit dividercircuit 380 provides frequency divided processed drum tach pulses andreset pulses on line 182 and 186, respectively, at the completion ofevery fourth revolution of the track scanning head 30, or at a frequencyof 15 Hz in a 60 Hz field rate standard and at a frequency of 12.5 Hz ina 50 Hz field rate standard.

Since the reset current pulses will be provided to the integrator 134after every four revolutions of the head 30, the integrator will providea head deflection ramp signal, lasting for four head revolutions betweenconsecutive reset current pulses, that deflects the movable element 32 adistance in the reverse direction corresponding to the distanceseparating four adjacent track centers. Therefore, upon the occurrenceof the frequency divided processed drum tach pulse on line 182, alllevel detectors 156, 158 and 550 are conditioned by the signal level online 66 exceeding the threshold levels established for the latches toprovide signals to the D input of the latches 170, 172 and 552,respectively, that enable the following associated AND gates 140, 142and 554 to pass frequency divided reset pulses when received over line186. For all color frame still motion modes, regardless of thetelevision signal standard, a fixed head reset determining thresholdvoltage level is provided on line 558 extending to one of the inputs ofthe level detector 550 corresponding to a head deflection in the reversedirection equal to the distance separating the centers of four adjacenttracks.

As described hereinabove, the three reset pulses passed by the AND gates140, 142 and 554 and converted by resistors 146, 148 and 556 to theappropriate pulse current levels are added together on line 132 toproduce a four track forward reset signal at the input of the integrator134. The four track forward reset current signal causes the headdeflection signal on line 66 to be reset and, thereby, effect a fourtrack forward deflection of the movable element 32 after eachreproduction of a four field sequence. In this manner, either an NTSC orSECAM color (depending on the signals being reproduced) still motionimage is provided by the record/reproduce apparatus.

For PAL standard color frame (composed of eight consecutive televisionfields) still motion operating modes, a level detector 560, togetherwith associated latch 562, AND gate 564 and current forming resistor 566connected to the output of AND gate 564 function together to provide anadditional four track forward reset current pulse over line 132 to theintegrator 134. To form the four track forward reset current pulse froma single reset pulse passed by AND gate 564, the impedance value of thecurrent forming resistor 566 is selected to be one-quarter the value ofresistors 146 and 148. In this still frame mode, AND gates 140, 142 and554 also cause a four track forward reset current pulse to be providedover line 132, which is added to the additional four track forward resetcurrent pulse to form an eight track forward reset current signal foreffecting a repositioning of the head 30 after eight revolutions. Theintegrator 134 responds to the eight track forward reset current singalon line 132 to cause the repositioning of the movable head 30 to thetrack containing the first field of a repetitively reproduced eightfield PAL color frame sequence after every eight revolutions of thehead. This is accomplished by placing the movable contact of the switch384 at the output of the eight bit divider circuit 380 (FIG. 10c) inposition 4. With the switch 384 so positioned, the eight bit dividercircuit 380 provides frequency divided processed drum tach pulses andreset pulses on lines 182 and 186, respectively, at the completion ofevery eighth revolution of the track scanning head 30, or at a frequencyof 6.25 Hz in a 50 Hz field rate PAL standard.

Since the reset current pulses will be provided to the integrator 134after every eight revolutions of the head 30, the integrator willprovide a head deflection ramp signal, lasting for eight headrevolutions between consecutive reset current pulses, that deflects themovable element 32 a distance in the reverse direction corresponding tothe distance separating eight adjacent track centers. Therefore, uponthe occurrence of the frequency divided processed drum tach pulse online 182, all level detectors 156, 158, 550 and 560 are conditioned bythe signal level on line 66 to provide signals to the D input of thelatches 170, 172, 552 and 562 respectively, that enable the followingassociated AND gates 140, 142, 554 and 556 to pass frequency dividedreset current pulses when received over line 186. For the PAL colorframe still motion mode, a fixed eight track reverse reference thresholdvoltage level is provided over a line 572 extending to one of the inputsof the level detector 560. As described hereinabove, the four resetpulses passed by the AND gates 140, 142, 554 and 564 and converted byresistors 146, 148, 556 and 566 to the appropriate current pulse levels,are added together on line 132 to produce an eight track forward resetsignal at the input of the integrator 134. The eight track forward resetsignal causes the head deflection signal on line 66 to be reset and,thereby, effect an eight track forward deflection of the movable element32 after each reproduction of an eight field PAL color frame sequence.In this manner, a PAL color frame still image is provided by therecord/reproduce apparatus. It should be appreciated that when therecord/reproduce apparatus is not operated to reproduce multiple fieldstill motion displays, the variable threshold reference source 126 isset to place disabling signals on lines 558 and 572 extending to one ofthe inputs of the level detectors 550 and 560, respectively. Asdescribed hereinbefore with respect to the function of level detector154 in the other operating modes of the record/reproduce apparatus, thisprevents the level detectors 550 and 560 from enabling their associatedAND gates to pass reset pulses to the line 132 (FIG. 3) that controlsthe resetting of the integrator 134.

The modified portion of the automatic head tracking servo circuitryshown in FIG. 10d cooperates with the modified portion of the servocircuitry shown in FIG. 10c to provide the required reset pulse signalfor the various still frame operating modes described hereinabove toprevent the servo system of the apparatus from locking in the ambiguousstates described hereinabove. In this respect, line 574 extends from theNAND gate 345 (FIG. 10a), which provides the aforedescribed latch holdsignal lasting for a period of ten reset pulses. The unmodified headtracking servo circuitry shown in FIGS. 10a and 10b is arranged toprovide a latch hold signal only to the set terminal of the latch 172because the servo circuitry is arranged to produce still motion displaysfrom a single repetitively reproduced field and this requires only a onetrack forward reset of the head 30. In a monochrome frame still motionmode, a 2 track forward reset signal is required because two consecutivefields are repetitively reproduced. To provide a 2 track forward resetsignal for the ten-reset-pulse period, a switch 576 is closed whenoperating in the monochrome frame still motion mode so that the setterminal of the latch 170 also receives the latch hold signal placed online 574. Since both latches 170 and 172 are placed in the set state forthe ten-reset-pulse period, their associated AND gates 140 and 142,respectively, are enabled for the same period, which, as describedhereinbefore, results in the generation of a 2 track forward resetcurrent signal on the line 132 extending to the input of the integrator134.

In either an NTSC standard or SECAM standard color frame still motionoperation, a 4 track forward reset current signal is required becausefour consecutive fields are repetitively reproduced. To provide a 4track forward reset current signal for the ten-reset-pulse period, bothswitches 576 and 578 are closed when operating in all color frame stillmotion modes so that the set terminals of the latches 170 and 552 alsoreceive the latch hold signal placed on line 574. Since the threelatches 170, 172 and 552 are placed in the set state for theten-reset-pulse period, their associated AND gates 140, 142 and 554,respectively, are enabled for the same period, which, as describedhereinbefore, results in the generation of a 4 track forward resetcurrent signal on line 132.

In the PAL color frame still motion mode, an 8 track forward resetcurrent signal is required for the ten-reset-pulse period because eightconsecutive fields are repetitively reproduced. To effect the generationof an 8 track forward reset current signal for the ten-reset-pulseperiod, a switch 580 is also closed so that the set terminal of thelatch 562 also receives the latch hold signal placed on line 574. Sinceall of the latches are placed in the set state for ten-reset-pulseperiod, their associated AND gates are enabled for the same period,which, as described hereinbefore, resutls in the generation of an 8track forward reset current signal on line 132.

The exemplary embodiment of the automatic head tracking servo circuitryshown in FIGS. 10a and 10b have provisions for performing other specialfunctions in accordance with certain input signals received. Forexample, because the head positioning error signal typically is a lowrate error signal in normal speed operating modes, it is advantageous tosample the synchronous detector output signal on line 80 during theintermediate portion of the scan of a track by the rotating head 30. Forthis purpose, a normally open switch 122 (FIG. 10b) is interposed in theline 80 of the head position error feedback path extending between theoutput of the synchronous detector 78 and the input of the integrator134. During normal speed modes, the AUTO TRK signal on input line 283enables an NAND gate 429 to pass a DC GATE signal provided on input line430. The DC GATE signal is derived from the 60 Hz drum tach signal andis delayed to occur intermediate of consecutive drum tach signals. TheDC GATE signal is passed by the NAND gate as a low level pulse signallasting for about 4 MSEC. If the automatic head tracking circuit shownin FIGS. 10a and 10b is switched on, the following low level AND gate431 issues a high level pulse corresponding in duration to the DC GATEsignal to enable the switch 122 to pass the low rate head positioningerror signal to the integrator 134, which responds by adjusting the DClevel of the head position servo correction signal provided on line 68extending to the second summing circuit 69 (FIG. 12).

The automatic head tracking servo circuit also includes means to disableit in the event the drum portion 22 of the tape guide drum assembly 20(FIG. 4), hence, movable head 30 is not rotating. If the drum portion 22is not rotating, a low logic signal level is placed on input line 434(FIG. 10b) that is processed by the logic circuitry 111 of the automatichead tracking servo circuit to provide disabling signals that openswitches 312 and 316.

Frequently, a recorded tape will be played back on differentrecord/reproduce apparatus. In many instances, the recording apparatusand reproducing apparatus will be characterized by differentialgeometric head-to-tape tracking trajectory variations that lead tointerchange errors. Because such geometric variations are random innature, severe mistracking conditions can occur during reproductionoperations. To facilitate the control of the movable head 30 so that thetracks of such recordings can be precisely followed, a switching means433 is included in the dither oscillator 60 that is controllable by anoperator to double the amplitude of the dither signal provided to themovable element 32 via the line 62. The twice amplitude dither signal isselected by an operator causing, through suitable control device, a highlogic level AST RANGE signal to be placed on input line 435. Applying atwice amplitude dither signal to the movable element 32 has the effectof increasing the servo capture gain of the head tracking servo circuit,thereby extending the servo capture range.

As previously described herein, the movable element 32 has a limitedrange over which it can be deflected. For record/reproduce apparatuspreviously constructed for commercial applications, this range has beenselected to be a distance corresponding to ±1.5 times the distanceseparating adjacent track centers. To facilitate tracking of therecorded information without the introduction of undesirable disturbingeffects in the reproduced signals when the apparatus is operated in theaforedescribed extended range, the apparatus includes an automatic tapeslew drive command signal generator 436, which is responsive to thecombined DC error plus head deflection signal present on line 66a togenerate one or more track slew tape drive commands on the appropriateone of the output lines 437 and 438. These lines extend to the capstanmotor drive amplifier 220 for coupling the tape slew commands thereto.Because of the severe mistracking conditions encountered in the extendedrange operating mode, the movable element 32 frequently is displacedtowards one of its limits. To maintain the movable element within itsdeflection range in such mode of operation, the generator 436 isarranged to provide a slew command to the capstan motor drive amplifier220 whenever the deflection of the movable element 32 exceeds ±15% ofthe distance separating adjacent track centers. In this manner, themovable element 32 is maintained within its deflection range limits. Inthe event the movable element 32 exceeds the 15 % deflection limit inthe forward deflection direction, the head deflection thresholdreference level associated with the tape slew reverse control isexceeded and SLEW REV commands are provided by the generator 436 overoutput line 438 to slow down, or reverse the direction of the transportof the tape 36, whichever is needed. SLEW FWD commands are provided bythe generator 436 over line 437 when the movable element 32 exceeds the15% deflection limit in the reverse deflection direction.

Turning now to FIGS. 11a, 11b and 11c, there is shown one embodiment ofspecific circuitry that can be used to carry out the operation of aportion of the transport servo illustrated by the block diagram of FIG.8. The portions of the tape transport servo shown in the block diagramof FIG. 8 not included in FIGS. 11a, 11b and 11c are those previouslyidentified, namely, the control track phase comparator 270, controltrack error window detector 276 and color frame detector 280, as beingincluded in typical helical scan video record/reproduce apparatus thatprovide signals used by the tape transport servo to carry out itsoperations. Furthermore, the transport servo is arranged to control thetransport of the tape 30 so that the record/reproduce apparatus can beoperated to record and reproduce television signals of both 50 Hz and 60Hz line standards. The 50/60 Hz signal level placed on the input line338 sets the transport servo in the operating condition necessary forthe television signal standard of concern. The specific circuitry shownin FIGS. 11 a, 11b and 11c is arranged to control the transport of thetape when recording or reproducing NTSC television signals for PAL andSECAM television signals, certain timing provided by the transport servocircuitry shown in FIGS. 11a, 11b and 11c is preferably changed toaccount for differences in the timing associated with such signals,which changes will be readily apparent from the following description ofthe transport servo and, hence, need not be described in detail herein.

The record/reproduce apparatus, for which the transport servoillustrated by FIGS. 11a, 11b and 11c is constructed, has severaloperating modes that can be selected through the operation of operatorcontrols, with each operating mode requiring a different response fromthe illustrated transport servo. In slow/still operating modes, anoperator initiated slow/still mode command (SLOW) is placed on inputline 353 (FIG. 11a) and is coupled thereby to condition the logiccircuitry 224 (FIG. 8) so that the transport servo provides the requiredcontrol of the transport of the tape 30. At tape transport speeds lessthan 95% normal speed, the transport servo provides velocity control ofthe transport of the tape 30.

With reference to FIG. 11a, velocity control of the tape transport atless than normal speeds during slow/still operating modes is provided bythe variable slow motion control circuitry 240. The control circuitrygenerates the variable capstan drive for driving the capstan motor 202(FIG. 8) within a speed range from a very slow speed up to a maximum ofabout 95% of normal speed. The operation of the entire circuitry 240 isdescribed in detail in the afore-mentioned application of Mauch, Ser.No. 874,739. The variable width pulses generated by the variable slowmotion control circuitry 240 for driving the capstan motor 202 invelocity control servo modes of operation at speeds below the cross-overvelocity of about 1/5 normal speed are provided on line 242 in responseto the pulse reference signal received over input line 355, which is alevel and gain adjusted signal corresponding to the setting of thepotentiometer 240' (FIG. 8). At tape transport speeds below thecross-over velocity, a velocity drive control circuit 356, which iscoupled to examine the output of the frequency discriminator circuit210, issues a command over one of the control lines 230a that causesswitch means 226 to connect the pulse drive output line 242 of thevariable slow motion control circuitry 240 to the motor drive amplifier220 (FIG. 8) via line 218 and disconnects the capstan and control trackphase comparators 212 and 270 from the capstan motor drive circuitry.This circuit condition corresponds to the block diagram illustration ofFIG. 8 with the movable contact means 228 of the switch means 226 inposition 1.

The tachometer input appears on line 208 in the upper left corner ofFIG. 11a and is coupled for processing by tachometer input processingcircuitry 352, the processed capstan tachometer signal being coupled tothe input of the velocity loop frequency discriminator 210. The velocityloop frequency discriminator is operatively connected to a velocity looperror amplifier 354 and the velocity drive switch control circuit 356 toprovide velocity control over the transport of the tape 36. When thepotentiometer 240' (FIG. 8) of the variable slow motion controlcircuitry is adjusted to cause the capstan 200 (FIG. 8) to be driven totransport the tape 36 at speeds within the range of about 1/6 to 1/3normal speed, the velocity drive switch control circuit 356 responds tothe velocity related signal level provided by the frequencydiscriminator 210 and a following integrating circuit 357 by issuingcommands over control line 230a that toggles the switch means 226respectively between its two conditions. As described in detail in theaforementioned Mauch application, Ser. No. 874,739, toggling switchmeans 226 alternately couples to the capstan motor drive amplifier 220(FIG. 8) via line 218 the pulse drive signal present at line 242 of thevariable slow motion circuitry 240 and the analog drive signal presenton line 217, which is generated by the frequency discriminator 210 andassociated circuitry in response to the tape velocity related signal inthe form of processed capstan tachometer signals and a velocityreference signal generated by the velocity reference circuitry 250. Attape speeds in excess of 1/3 normal speed, the switch means 226 ismaintained in a condition to couple the drive signal generated by thecooperative action of the velocity reference circuit 250 and thefrequency discriminator 210. In these higher slow motion operatingmodes, the tape transport speed is controlled by the potentiometer 240'(FIG. 8), which is connected to provide the slow speed control signal oninput line 363. A command placed on command line 252a by the logiccircuitry 224 enables a switch means 362 to permit the slow speedcontrol signal to be coupled to establish a voltage level at the inputof an integrating circuit 359 of the velocity reference circuit 250 thatcorresponds to the setting of the potentiometer 240'. The output signalprovided by the velocity reference circuit is coupled to one input of asumming junction formed by a summing amplifier 361 for subtraction withthe velocity feedback signal, generated by the frequency discriminator210 and coupled to another input of the summing amplifier 361. Anydifference between the signals represents a tape velocity error and iscoupled as a velocity error signal to the output line 217 to thevelocity loop error amplifier 354 for application to the capstan motordrive amplifier 220 (FIG. 8) via switch means 226 and line 218.

The transport servo also provides velocity control over the transport ofthe tape 30 whenever the record/reproduce apparatus is operated toaccelerate the tape to enter a normal reproduce mode of operation. Anormal speed reproduce mode of operation is initiated by the operatoractivating controls that places a PLAY mode command signal on line 364,which causes the logic circuitry 324 to place the command on the commandline 252b that results in the generation of a voltage step on line 363.The integrating circuit 359 responds to the voltage step by generatingon its output line 254 a ramp signal of a fixed, selected interval forapplication to the summing amplifier 361. As described hereinbefore, theoutput of the summing amplifier is coupled to drive the capstan motor202 and, when the summing amplifier 361 receives a ramp signal from theintegrating circuit, the capstan motor 202 is caused to accelerateaccording to the slope of the ramp signal.

The tachometer reference divider 260 is shown in FIG. 11a and iscontrolled by control line 262 which has a low logic level when the tape30 is transported at the 95% of normal tape speed and a high logic levelwhen it is transported at 100% of normal tape speed, with the line 252extending from logic circuitry shown in FIG. 11c. The transport servo isplaced in the capstan tach phase lock mode by an operator initiated PLAYmode command coupled to input line 364. Initially, the transport servologic circuitry places the transport servo in the aforedescribedacceleration mode of operation for a predetermined acceleration intervalof about 0.5 sec., if the tape 30 is stopped at the time the PLAY modecommand is received and a correspondingly shorter time if the tape isalready in motion when the PLAY command is received. The interval is setto provide sufficient time for the servo to establish the desiredvelocity controlled servo lock condition. A one-shot 365 provides asettling delay of about 0.3 sec. after control of the transport servo isswitched to the capstan tach phase comparator 212. Upon initiation ofthe 0.3 sec. settling delay interval, the logic circuitry issues acommand over one of the control lines 230b to close the switch 232a(FIG. 11c) and, thereby, allow the capstan phase comparator 212 to becoupled to control the capstan drive. In addition, the logic circuitryplaces a low logic level on line 262, which causes the variable divider260 to generate a 95% normal speed mode servo reference signal from the64 H clock on input line 264, which reference signal is coupled by line258 to the input of the capstan tachometer servo loop phase comparator212 (FIG. 11c). Any phase error between the capstan tack signal receivedon input line 208 and the 95% normal speed mode servo reference signalis detected by the phase comparator 212, which responsively provides aproportionate voltage level signal on the input line 369 of a tachometerlock error amplifier 360 shown in FIG. 11c. The output of the tachometerloop error amplifier 360 is coupled by the closed switch 232b (whichcorresponds to the movable contact means 234 of the switching means 232shown in FIG. 8 being in position 2) to line 244 that extends to thesumming junction 214 and, as described hereinbefore, eventually to thecapstan drive amplifier via line 218 for driving the capstan 200 underthe desired capstan tach phase lock conditions.

Servo control of the transport of the tape 30 is switched from 94%normal speed capstan tach phase lock mode to the 100% normal speedcapstan tach phase lock mode when the initial color framing is complete,i.e., the correct field sequence for proper color frame conditions isreproduced, and the detected control track error is within theaforedescribed ±10% window defined by the control track servo referencesignal, so that the initial color frame condition will not be lost whenservo control is switched. The logic circuit portion 374 (FIG. 11b)primarily coordinates the acquisition of the correct field forreproduction operations and controls the switching of the transportservo system from the capstan tach phase lock mode to the control trackphase lock mode. When the initial color frame operation performed withrespect to the reproduced control track signal is complete, the colorframe detector 280 (FIG. 8) provides a high logic signal level,designated CT COLOR FRAME, at its output on line 284a (FIG. 11b), whichextends to a pair of cascaded D latches 373 included in the portion 374of the logic circuitry. Also, a studio reference signal, designated CTREF, is coupled by the line 248b to the clock input of the first of thecascaded D latches 373. The CT REF signal is a 30 Hz logic levelchanging signal having a low-to-high logic signal level transitiondisplaced in time relative to the occurrence of the 30 Hz studio controltrack reference by an amount equal to 1/60 sec. This signal serves toclock the level of the CT COLOR FRAME signal present on line 248a to thesecond of the cascaded D latches. When the control track error signalpresent on line 274 at the output of the control track phase comparator270 is within the foredescribed ±10% error window, the control trackerror window detector circuit 276 (FIG. 8) generates a high logic signallevel, designated CT WINDOW, on line 278 extending to the clock input ofthe second of the cascaded D latches 373. If this occurs following theestablishment of the proper color frame production conditions, thelow-to-high signal level transition of the CT WINDOW signal clocks theproper complementary logic signal levels at the output of D latchcircuitry 373. These signals condition the following logic circuitry tocause a high logic signal level to be placed on line 262, which sets thevariable divider 260 to generate a 100% normal speed mode control trackservo reference signal. This servo reference signal is coupled to line258 that extends to the input of the capstan tachometer servo loop phasecomparator 212. Because at this time the tape 30 is being transported ata speed corresponding to 95% of the normal speed, the capstan tach phasecomaprator 212 generates an error signal that is processed by thetachometer lock error amplifier 360 to provide a corresponding capstanmotor drive signal for accelerating the transport of the tape 36 to thenormal speed characteristic of normal motion reproduction operations.After a settling interval of about 0.6 sec. determined by the activeinterval determining time constant of the one-shot 371, the logiccircuitry 224 generates a CT SERVO command over control line 230c (FIGS.11c) that closes switch 232b while simultaneously opening the switch232a by terminating the switch closure command on line 230b. Placingswitches 232a and 232b in the aforedescribed states corresponds to themovable contact means 234 of the switching means 232 shown in FIG. 8being in position 3. Opening the switch 232a removes the capstan tachphase comparator 212 from tape transport servo loop. The closed switch232b couples the control track error signal generated by the controltrack phase comparator 270 on the line 274 to the summing junction 214and, as described hereinbefore, eventually to the capstan motor driveamplifier 220 (FIG. 8) for providing the drive to the capstan 200 underthe desired control track phase lock conditions.

As previously discussed herein, the control of the tape transport servois coordinated with the control of the automatic head tracking servocircuitry shown in FIGS. 10a and 10b. This coordination is accomplishedprimarily by the portion 370 of the logic circuitry shown in FIGS. 11band 11c, which couples the appropriate coordinating control signals tothe automatic head tracking servo circuitry over lines 372a, 372b, 372cand 372d. When the apparatus is operating in the slow/still mode, thelogic circuitry portion 370 places a log logic signal level on line 372athat enables the automatic head tracking servo circuitry to control theposition of the movable head during slow/still modes of operation. Whenthe apparatus is operating in the capstan tach phase lock mode duringboth the 95% and 100% normal speed modes, the logic circuitry portion370 places a low logic signal level on line 372a after the control ofthe transport servo is switched to the capstan tach phase lock mode.This signal is designated AST TACH and is coupled by line 372b tocondition the automatic head tracking servo circuitry to control theposition of the movable head during capstan tach phase lock mode thatoccurs during the 95% and 100% normal speed operating modes. When thetransport servo is commanded to accelerate the tape 36 to a speedcorresponding to 100% normal speed, the logic circuitry portion 370places a low logic level pulse 503 (FIG. 16) on line 372c, which has aduration of about 0.6 sec. This signal, designated 100% TACH, is coupledto the automatic head tracking servo to condition it for controlling theposition of the movable head at the completion of the initial capstantach phase lock mode portion of the 100% normal speed mode. As describedhereinbefore, the presence of the 100% TACH pulse signal at the input ofthe inverter 450 (FIG. 10a) disables the level detector 158 byconditioning the associated open collector gates of the variablereference threshold level source 126 to place a high voltage level online 196. Consequently, only the level detectors associated with 1 TRKREV and 1 TRK FWD threshold levels are enabled to control the positionof the movable head 30 during the 100% normal speed mode. Furthermore,the trailing edge 503a (FIG. 16) of the 100% TACH pulse enables thecolor frame verification circuitry 340 to respond to the FIELD MISMATCHsignal present at the one of the inputs of the AND gate 441 toreposition the movable head 30 a distance in the appropriate directioncorresponding to the distance separating adjacent track centers in theeven a field mismatch is detected at the time control of the transportservo is switched to the control track phase comparator 270 (FIG. 8).

Synchronous production of the recorded signals under automatic headtracking servo conditions is commenced in response to the provision ofthe AUTO TRK signal on line 372d at the conclusion of the 100% TACHsignal if an AST AUTOTRK enabling mode command signal is received oninput line 358 as a result of an operator initiated control switch. TheAUTO TRK signal occurs simultaneously with the presence of the CT SERVOsignal on the control line, which as described hereinabove, inserts thecontrol track phase comparator 270 in the transport servo forcontrolling the transport of the tape. The AUTO TRK signal is coupled tomode control line 285 of the automatic head tracking servo to conditionit for controlling the movable head during the normal speed mode aspreviously described herein.

The exemplary embodiment of the transport servo shown in FIGS. 11a, 11band 11c have provisions with performing other special functions inaccordance with certain input signals received. For example, the logiccircuitry 224 includes means to inhibit sequencing of transport servo ifcertain operating conditions are not satisfied. If the drum portion 22is not rotating, hence, record and reproduce operations not beingcarried out, a DRUM OFF high logic signal level is provided by theapparatus on input line 368 (FIG. 11a) that inhibits the logic circuitrysequence. Similarly, in the event reproduced video is not present, theapparatus inhibits the logic circuitry sequence by removing an enablinghigh logic level RF PR signal from the input line 375 (FIG. 11b). If thevideo signal is being reproduced from a tape that does not include arecorded control track signal (or the control track signal ismomentarily lost), the logic circuitry sequence is interrupted at (orreturned to) the 95% normal speed mode condition and servo control ofthe transport of the tape 30 is retained by the capstan tach phasecapacitor 212 as a result of the removal of the high logic level CT PRsignal from input line 376 (FIG. 11b). Automatic resumption of thetransport servo sequencing occurs if the switch 293 (FIG. 11b) has itsmovable contact in the AUTO position. If the switch 293 is in the MANposition, resequencing of the transport servo is initiated by causingone of mode commands to be placed or an input line to the transportservo.

The transport servo is also arranged to permit control of the transportof the tape 30 with respect to a remotely occurring event, such as therecording on a remotely located record/reproduce apparatus of the videosignal reproduced by the record/reproduce apparatus controlled by theillustrated transport servo. Program editing is an example of this. Insuch operations, the transport of tape 30 must be carefully controlledrelative to the transport of the remotely located tape so that thereproduction of the video signal from the tape 30 is initiated at thedesired instant. To release the transport servo to remote control, anoperator initiated low logic signal level, designated TSO mode command,is placed on the input line 377 (FIG. 11b). The logic circuitry respondsto the TSO mode command signal by placing the transport servo in thevelocity servo mode and enabling the tape speed override circuitry 378(FIG. 11c) to couple on external velocity reference signal to the inputof the summing amplifier 361 (FIG. 11a) for comparison with the velocityfeedback signal generated by the frequency discriminator 210. Thusly,the tape 30 is transported at a speed determined by the externalvelocity reference signal present at the input line 379 of the tapespeed override circuitry 378.

Reverse tape drive operations are controlled by the transport servo bycoupling operator initiated mode command signals, designated REV JOGENABLE and REV JOG SWITCH, to the input lines 290 and 291, respectively.The generation of these two signals is initiated by adjusting thepotentiometer 240' (FIG. 8) to provide reverse velocity drive. Signalprocessing circuitry, like that provided for processing the PULSE REFand SLOW SPEED CONTROL signals, generates the REV JOG ENABLE and the REVJOG SWITCH signals. The REV JOG SWITCH mode command signal is coupled toplace the capstan motor 202 in the reverse drive operating condition, aslong as the reverse tape velocity is less than about 1/3 normal tapespeed. The REV JOG ENABLE mode command signal conditions the variableslow motion control circuitry 240 to provide reverse tape motionvelocity control in the same manner as described hereinbefore withrespect to forward tape motion velocity control at reverse tape speedsless than about 1/3 normal tape speed.

From the foregoing description, it should be appreciated that a methodand apparatus has been described which is particularly adapted for usewith a video record/reproduce apparatus of the type which has atransducing head that is movable to automatically follow a track duringthe transfer of information with respect to the record medium and whichcan then move the transducing head to the appropriate track dependingupon a mode of operation of the apparatus. By uniquely controlling thegain of the feedback signal detector of the head tracking circuitry,nondisruptive, noise free transfers of video information can bemaintained even though wide variations in the strength of the feedbacksignal occur during head tracking control. The resulting advantages aremost evident in the absence of disturbing effects in the transferredvideo information during the mode transitions, which are importantoperational considerations in commercial broadcasting of televisioninformation where such problems are avoided wherever possible.

It should be understood that although preferred embodiments of thepresent invention have been illustrated and described, variousmodifications thereof will become apparent to those skilled in the art;and, accordingly, the scope of the present invention should be definedonly by the appended claims and equivalents thereof.

Various features of the invention are set forth in the following claims.

We claim:
 1. In a video record/reproduce system using high frequencysignals in the form of an amplitude modulated RF envelope of frequencymodulated carrier, an automatically calibrated envelope detector circuitcomprising:envelope detector means for generating an output proportionalto the peaks of the amplitude modulated RF envelope; and feedback loopmeans responsive to selected envelope recurrent periods of knownamplitude level for automatically establishing a selected referencelevel during the recurrent periods to continuously servo the gain of theenvelope detector circuit to maintain a constant voltage change betweensaid reference level and an RF envelope without amplitude modulation. 2.The circuit of claim 1 wherein the feedback loop means is referenced toa selected voltage level during the RF envelope without amplitudemodulation, to servo the gain and define the constant voltage change. 3.The circuit of claim 2 further including variable gain amplifier meanscoupled to the envelope detector means and responsive to the feedbackloop means to correspondingly vary the gain thereof.
 4. The circuit ofclaim 3 wherein the feedback loop means includes a capacitor selectivelychargeable to said reference level during the recurrent periods.
 5. Thecircuit of claim 4 further including circuit means coupled between saidcapacitor and the variable gain amplifier means during the RF envelopewithout amplitude modulation, for establishing the associated selectedvoltage level to servo the gain.
 6. The circuit of claim 5 furtherincluding timing means for directing the charging of the capacitor tothe reference level during the recurrent periods, and the establishingof the selected voltage level during the RF envelope without amplitudemodulation.
 7. The circuit of claim 6 wherein;the feedback loop means iscoupled from the envelope detector means output to the input of thevariable gain amplifier, the loop means including; said capacitorcoupled to the envelope detector means output; said circuit meansincluding, a differential amplifier for establishing the selectedvoltage level, and an RC network coupled from the differential amplifierto control the variable gain amplifier means; and switch means disposedto couple the capacitor to define the reference level during therecurrent periods, and alternately couple the capacitor to thedifferential amplifier during the RF envelope without amplitudemodulation.
 8. The circuit of claim 4 wherein the video record/reproducesystem employs a transducing head for scanning along a plurality ofadjacent tracks, and includes head supporting movable means foreffecting lateral displacement of the transducing head with respect tothe direction of the tracks in response to an oscillatory dither signalapplied thereto, wherein said RF envelope is amplitude modulated by saiddither signal and defines recurrent drop out intervals of zero amplitudeat 100% modulation and alternate envelope periods amplitude modulatedabout a nominal envelope amplitude, said lateral displacement of thetransducing head being detected to define a head tracking error signalindicative of the amount and direction of the head displacement from itsoptimum position, wherein;said envelope detector means detects theamplitude of the RF envelope as modulated by the dither signal inresponse to the head displacement; said feedback loop meansautomatically establishes said selected reference signal during the zeroamplitude drop out interval, and the selected voltage level during thealternate envelope periods, to continuously servo the gain of theenvelope detector circuit to maintain said content voltage change;whereby variations therefrom in the output of the envelope detectorcircuit represent the lateral displacement of the transducing head fromits optimum position.
 9. The circuit of claim 1 further including;meansdisposed prior to said envelope detector means for generating a knownamplitude level of RF envelope during the recurrent periods thereof. 10.The circuit of claim 9 wherein the means for generating the knownamplitude level of RF envelope includes diode modulator means forgenerating 100% modulation of the RF envelope.